RÉSONAANCES

New Year, Same Old Song

2012-01-31T16:26:00.010+01:00

Today the CMS collaboration revealed several new analyses based on the full dataset the LHC collected in 2011. As usually, the recurrent theme was "no-significant-excess-was-found". Here is a selection of the most interesting searches and limits.

t' searches
In this analysis, filed under "4th generation", one looks for a heavier copy of the top quark: a fermionic particle with charge 2/3 produced in pairs and decaying to one b-quark and one W boson. We sort of know by now there is no 4th generation of quarks and leptons in nature, nevertheless this search is relevant to more interesting new physics models. For example, in a large class of little Higgs and composite Higgs models the fermionic partner of the top quark decays as t' → b W about half of the time. The current limit on the t' mass assuming 100% branching fraction for the t' → b W decay is 525 GeV. For little Higgs et al. the limit is slightly weaker, slightly above 400 GeV (due to the smaller branching fraction) but that is also beginning to feel uncomfortable from the point of view of naturalness of these models.

W' searches
This time the target is a heavy cousin of the W boson, decaying to one lepton and one neutrino. Unlike in the former case, there is no compelling theoretical reasons for such a creature to exist. However they represent a characteristic and clean signature that is fairly straightforward to look for: an energetic electron or muon accompanied by missing energy from a neutrino. To tell W' from the ordinary W boson one looks for events with a large transverse mass (for an on-shell particle whose decay products include a neutrino the transverse mass is less than the particle mass). Intriguingly, in the muon channel an outlier event with a very large transverse mass of 2.4 TeV is observed in the data. Of course, most likely it's just a fluke, but in any case it'll be interesting to see what ATLAS has in store.

t-tbar resonance searches
This search targets heavy (more than 1 TeV) particles decaying to a pair of top quarks, a signature very common in models with a new strongly interacting sector, like composite Higgs or the Randall-Sundrum model. Such a particle would produce a bump in the invariant mass spectrum of t-tbar pairs, which are otherwise copiously produced at the LHC. Top quark decays most often to 3 hadronic jets, but for a heavy mother resonance the daughter top quarks move so quickly that their decay products merge into one fat jet. Therefore this search relies on fancy modern techniques of studying substructure of jets, in order to identify closely packed jets that could originate from a fast moving top quark. No resonance is observed in the t-tbar spectrum. What is interesting is that the LHC sensitivity now reaches the cross sections predicted by popular versions of the Randall-Sundrum model, excluding Kaluza-Klein gluons lighter than about 1.5 TeV. My guess is that the explanation of the Tevatron anomalous top forward-backward asymmetry in terms of heavy KK gluon is now dead and gone.

SUSY searches
The only vanilla SUSY search updated with the full 2011 dataset is the one in the Z+jets+missing energy channel. This is not the first place you'd look for supersymmetry (that would be jets+MET); this search is relevant to a subset of models where a cascade of neutralinos and gravitons produces, often enough, an shell Z boson. Therefore the limits on the gluino mass are not stunning: 600-900 GeV depending on how squeezed is the SUSY spectrum. More spectacular SUSY limits are probably saved for the Moriond conference in about 1 month from now.

For more details, more models, more limits, and more disappointment have a look at the slides or the original summary notes on the CMS wiki page.

Elsevier

2012-01-31T02:04:00.006+01:00

The online initiative to boycott the Elsevier publishing company is gaining momentum. For particle theorists, signing to it does not take much. In our community, publishing in Elsevier has been considered, since several years already, on the same footing as farting at the dinner table: technically not forbidden, but somewhat disqualifying. It is heartening that this notion is now spilling outside our little world into large areas of mathematics, physics, and biology. This offers a realistic prospect for a change.

For those born yesterday, we resent Elsevier for:

charging exorbitant prices,
bundling their offer into all-or-nothing packages,
supporting SOPA/PIPA/RWA to the end of restricting freedom of information.

This adds up to the past record of setting up fake-peer-review medical journal, keeping the Chaos, Solitons and Fractals sham far too long, legal bullying, and supporting arms trade. Based on that list, you might object to the boycott by arguing that Elsevier is not infinitely more evil than an average publishing company. That's true, however, I personally view it as only a first step to reforming scientific publishing. Scientists, although modern in many respects, got entangled in a completely anachronic and very inefficient system of distribution and evaluation of their output. This could and should be changed. So, hitting Elsevier would be a largely symbolic move, akin to the storming of the Bastille. Once awareness is raised, it'll be time to march on Versaille and really change the balance of power.

As a particle physics blogger I should sadly note the LHC collaborations have been regularly publishing in Elsevier owned journals such as Physics Letters, Nuclear Physics, or Nuclear Instruments. I guess it's not purposely evil but simply inertia. So if you're an LHC experimentalist, please sign the pledge, and next time someone tries to submit to Elsevier please kick and scream; in case it doesn't help you may consider withdrawing your name from the publication. If the LHC could officially join the boycott, it would be a huge PR push for the initiative.

For further reading, see the posts of Tim Gowers, John Baez, Sean. And let's hope my historical analogy won't extend all the way to guillotining ;-)

SUSY and Higgs: romance or drama?

2012-01-25T01:25:00.009+01:00

At the beginning of 2012, particle physicists are in such a confusing state of mind: Higgs has been practically discovered but we're not allowed to celebrate yet. It's like when your football team is on top of the league, playing in the last round against a relegated team and winning 2:0 after the first half; nothing is decided yet, anything may happen, but... come on... So, to stay sane, most of us act as if the 125 GeV Higgs were a fact and work out the consequences.

In that vein, this post is about a complicated relationship between the 125 GeV Higgs and supersymmetry. There is this lore that SUSY predicts the Higgs mass below 130 GeV, and you might have heard people saying that the recent almost-discovery of the Higgs is an incredible success of supersymmetry. Well, strictly speaking, the number 130 GeV is taken out of my ass. Instead, with some degree of rigor, one can make the following 3 statements:

Minimal SUSY without fine-tuning predicts the Higgs mass close to the Z boson mass, that is about 90 GeV.
Minimal SUSY ignoring fine-tuning predicts the Higgs boson lighter than 160 GeV.
Non-minimal SUSY in general makes no predictions about the Higgs mass.

The last point is pretty obvious: once you agree to extend the minimal supersymmetric model (MSSM) then options become infinite. Even straightforward extensions of the MSSM, such as the NMSSM with one additional singlet field in the Higgs sector, allow one to cover the entire Higgs mass range up to almost a TeV. (You might be confused if you heard that the NMSSM predicts the Higgs mass below 140 GeV. That however is the case when the Higgs self-coupling is required to stay perturbative all the way up to the GUT scale, a strong and not particularly motivated assumption.)

The statement #1 on my list boils down to the fact that in the MSSM the quartic term in the Higgs potential (which fixes the Higgs mass, given its vacuum expectation value) is not a free parameter. Instead, supersymmetry ties the quartic coupling to the electroweak gauge couplings.
Up to 1-loop precision the Higgs mass is given by the formula:
(for vanishing A-terms, a large tanβ, and universal stop masses, and setting yt=1). In the first approximation one gets the famous bound m_Higgs ≤ m_Z. Thus, if the MSSM were for real, the Higgs should have been seen at LEP.

Only when supersymmetry is badly broken, that is when the top mass is much smaller than the mass of its scalar partner the stop, the one-loop logarithmic term can be large enough to raise the Higgs mass considerably above the Z boson mass. In particular, for the 125 GeV Higgs the tree-level and loop contributions must be, amusingly, almost exactly equal. The price for making the stop mass large goes under the name of fine-tuning. Since vacuum equations in the MSSM generically marry the SUSY scale to the weak scale, m_stop ~ m_Z , as soon m_stop >> m_top one needs to carefully tune the parameters of the theory so as to cancel various excessive contributions to the Z boson mass. This goes against the original motivation for supersymmetry which was precisely to exorcise fine-tuning.

This brings us the statement #2 on my list. When the fine-tuning issue is ignored, the scenario known as split supersymmetry (SS), the Higgs mass in the MSSM can be much larger than the Z boson mass. In the plot on the right (from this paper), you can see that the Higgs mass can reach 155 GeV for scalar SUSY partner masses at the GUT scale. From the same plot, one finds that the 125 GeV mass correspond to roughly 10 TeV squark masses. Thus, the almost-discovery of the 125 GeV Higgs at the LHC clearly points to Somewhat Split Supersymmetry (SSS) ;-)

All in all, the story of Higgs and SUSY is getting less like a Hollywood romance and more like a Ken Loach movie of hardship and misery. Of course, it is well known that 10 TeV squark masses are not an inevitable consequence of the MSSM and 125 GeV Higgs. Playing with another SUSY breaking parameter, the so-called A-term, the Higgs mass can be dialed to any desired value. When the A-term is judiciously chosen, the scalar top partners could even be at a few hundred GeV, well within the reach of last year's LHC run. See the violet band in the plot on the right. Thus a happy ending cannot be completely excluded at this point. However, more and more theorists are beginning to prepare an exit strategy, like ...nobody said SUSY had to show up at the LHC, maybe fine-tuning 1:1000 is not so bad, maybe SUSY is really at 10 TeV, etc... In a sense, this is right: from the theory point of view there is no fundamental difference between 1 in 100 and 1 in 1000 fine-tuning. Only a practical one, for LHC experimentalists :-)

To wrap up this inflammatory post: the point I was trying to make is that 125 GeV Higgs is not a successful prediction but rather a serious setback from the point of view of SUSY. In non-minimal SUSY any Higgs mass is possible. Minimal SUSY can accommodate any mass up to almost 160 GeV, depending on how much fine-tuning you're willing to accept; 125 GeV Higgs points to 10 TeV squarks, outside the LHC reach.

Five

2012-01-05T21:13:00.007+01:00

So, here's another candle on my cake. Resonaances has been hanging around the blogosphere for exactly 5 years now, gaining reputation for spreading unfounded rumors, for trying (in vain) to sound funny, and for (successfully) annoying everybody around. Traditionally, the New Year time is a lazy moment when bloggers take a pause and, rather than chasing news stories, post various summaries and wishlists. From me, as a birthday present, here's a list of 5 important questions in particle physics that likely will be answered by the end of 2012.

Is there a Higgs boson and why at 125 GeV?
Particle physicists are divided into those who think that Higgs is as good as discovered and those who think one should not say it loud. In any case, 2012 will go down in history books as the discovery year. Moreover, we will start learning something the Higgs couplings, thus kicking off with Higgs precision physics - the subject that will probably dominate particle physics for the next 2 decades.
Is there anomalous top quark forward-backward asymmetry?
There is this persistent anomaly at the Tevatron: the direction of motion of top quarks is statistically more forward than that of antitop quarks (where forward/backward is the direction of the proton/antiproton beam); the asymmetry is 10-15% larger than predicted by the state-of-art Standard Model calculations. This year we'll get more insight into this phenomenon thanks to analyses of the full Tevatron dataset. Moreover, the LHC will narrow down on the related observable called the charge asymmetry of top pair production, and see whether any discrepancy with the Standard Model appears. If it does, it's gonna be a beautiful year. On the other hand, if nothing unusual is seen by the end of this year, that will be a big blow to our hopes that new physics is lurking there.
Is there anomalous CP violation in the B meson sector?
Back in 2010 the D0 experiment found in their data the di-muon charge anomaly: a 1% excess of events with 2 negatively charged muons over those with 2 positively charged muons. The most appealing interpretation of that asymmetry was anomalous CP violation in B meson mixing, that is anti-B mesons turning into B-mesons more often than the other way around. However that interpretation is now under serious strain, because no CP violation has been observed by LHCb in the Bs meson decay to J/Ψ ϕ and to J/Ψ f0, and simultaneously no CP violation in the Bd meson sector has been seen in B-factories. This year LHCb should weigh in with another measurement of the difference of Bd and Bs meson CP asymmetries. That should sweep the floor once and for all, or, hopefully, open Pandora's box.
What went wrong in OPERA?
Most physicists, including those working for the OPERA collaboration, don't think that neutrinos are superluminal (because of theoretical and indirect experimental arguments). It seems likely that the bug explaining the 60 nanosecond shift of the arrival time of neutrinos sent from CERN to Gran Sasso will be found already this year. The talk in town is that more experimental work will be done soon, with Gran Sasso's experiments ICARUS and Borexino joining in the game with independent measurements of the neutrino speed. Is it the GPS? The clock? A delay in electronics? Or is it the magic mountain?
Is there supersymmetry below the TeV scale?
SPOILER ALERT: No. Well, so far we know for sure it's not there in its most popular incarnation, with gluino and squarks decaying via short cascades to much lighter stable neutralinos, thus producing a lot of missing energy. This year the net will be made much tighter, thanks to more data, the collision energy (likely) increased to 8 TeV, and many new analyses targeting more stealthy SUSY scenarios. By the end of this year surviving scenarios with squarks and gluinos below TeV will become collector's items, cherished for their rarity rather than beauty.

As a bonus, one more question that will be answered by the end of the year, and that is probably more pressing for a larger audience:

Will there be an Armageddon on December 21?
Not sure about it, but definitely the LHC should not be running on that day. So that, if the world ends, they won't blame us.

And to finish the party, here's the song that David Bowie wrote especially for today's anniversary of Resonaances:

2011 Recap

2011-12-31T18:10:00.012+01:00

What a year it was! It started with an earthquake, followed by rising tension. In the previous years I had to think hard to make up a decent blogging subject, this year it was enough to just sit and wait for the next breaking news. Here's a pick of the most important events in the particle world in the year 2011.

January: CDF finds new physics in tops
Measurements of the forward-backward asymmetry of the top pair production at the Tevatron have been showing interesting hints of new physics for a long time. CDF found that this asymmetry displays a very steep dependence of the energy of the colliding partons (that is on the invariant mass of the top pair) which represents a 3.4σ departure from the Standard Model predictions. Unfortunately, later on D0 did not confirm the steep mass dependence, and we need to wait more for the matter to be clarified.

April: CDF gives us goosebumps
CDF studied the invariant mass spectrum of jet pairs produced in association with a W boson, and found a bump near the 150 GeV dijet mass. After the subsequent update, the significance of the bump exceeded 4σ. It would be a clear evidence of new physics if not for the evil D0 who did not find any bump in their data. The origin of the bump will probably remain a mystery forever, much like the Roswell incident, because the Tevatron management is not very determined to resolve it.

April: Xenon100 does not find dark matter
Dark matter searches gain sensitivity at an impressive pace. Xenon100, currently the most sensitive detector for the vanilla dark matter particles, published the results based on the first 100 days of running. No luck so far. As a consolation, for each experiment that does not find dark matter there is one that does; this year CRESST joined the latter club, while CoGeNT announced an annual modulation of its detection rate. In any case, the hunt continues, it will take several more years before we may conclude we're looking in the wrong place.

April: Red Higgs Alarm
It was the first Higgs alarm this year: an internal ATLAS memo leaked to a blog was claiming a 4σ evidence for a 115 GeV Higgs in the γγ channel. The second time the alarm buzzed was after the EPS conference, when the unpublished combination of ATLAS and CMS was showing nearly 4σ evidence for a Higgs around 145 GeV. In both cases the evidence was quickly washed away by more data from the LHC. One positive thing we learned is that in the digital era collaboration secrecy is moot :-)

Summer: LHC bites deep into new territory
Around the EPS and Lepton-Photon conferences the LHC presented a number of new physics searches based on 1 inverse femtobarn of data. Alas, nothing exciting was found. SUSY, at least in its simplest incarnations, is being pushed above 1 TeV. And so are extra dimensions, technicolor, Z primes, dijet resonances, and any broader scenario that is not stealthy by design. Cold sweat is running down the backs of particle theorists...

September: OPERA goes superluminal
The OPERA collaboration stunned the world announcing that neutrinos produced at CERN arrive to their detector in Gran Sasso about 60 nanoseconds earlier than expected if they traveled at the speed of light. However the physics community remains unconvinced. There is indeed a number of very strong phenomenological and theoretical arguments that the OPERA result cannot be right. What do I think? At first I thought the pulse shape was the culprit, but that criticism was subsequently addressed by repeating the measurement with 3 nanosecond pulses. Now I think they should patent their set-up as a GPS synchronization tool :-)

September: US pulls the plug on Tevatron
It was like seeing an old dog put to death: you know it's for the better, but still it feels so sad. Of course, the Tevatron has not said the final word yet; we're still waiting for the collaborations to analyze the full data set. But the number of searches where the Tevatron can compete with the LHC is shrinking rapidly...

November: LHCb finds CP violation in charm
LHCb found that decays of D mesons into π+π- and K+K- violate the CP symmetry (at least one of them). We have seen CP violation in the K and B meson sectors, but the LHCb result was a big surprise: there was a lore that discovering CP violation in the charm meson sector at the 1% level would be a clear sign of new physics (although that is no longer so clear). Whatever it is, we learned something new about the world...

December: LHC glimpses Higgs
Higgs has been finally cornered and we think we're seeing the tip of his hat. Both ATLAS and CMS observe an excess of events in the γγ, ZZ and WW channels consistent with a Higgs boson of mass around 125 GeV. To be continued and ultimately resolved in 2012.

So raise your glasses, this one's over. According to the Mayans, the year 2o12 will be a bit shorter, but it shouldn't be less eventful ;-)

Sleeping with the Higgs

2011-12-19T06:48:00.006+01:00

All my life the Higgs was sort of a legend. Something like a unicorn: you know well how it's supposed to look like but you don't seriously expect to see it. Since last week it feels different. Of course, everybody from the CERN DG to common bloggers justly calls for caution and not jumping to conclusions. Of course, we've seen much larger excesses go away. Of course, there is statistics, likelihoods, look-elsewhere effects, and other attributes of civilization. But there's also the good old hunch. The latter, after digesting the totality of ATLAS and CMS findings, tells me there's about a 90% chance that the Higgs exists in the mass range 124-126 GeV. This high estimate follows from the remarkable consistency of the signal between ATLAS and CMS, and between different search channels. Moreover, that mass neatly fits into the ballpark suggested by electroweak precision fits. Last not least, most of the remaining Higgs mass range is either excluded or disfavored, except maybe around 119 GeV. In other words, I don't care much about the look-elsewhere correction because there isn't any elsewhere to speak of.

Thus, I'm slowly getting used to the idea of the Higgs becoming flesh and bones. On one hand this gives me thrills, as one of the greatest mystery of the universe is being solved in front of our eyes. At the same time I can see dark clouds at the horizon. There has been a well-founded hope that the Higgs would show some unexpected properties, thus opening us the doors to new physics. In the end, it would not make sense if a theoretical concept put forward 40+ years ago showed up in Nature in precisely the predicted form, would it? Yet what ATLAS and CMS are seeing looks dangerously close to the Standard Model Higgs: the signal is showing up everywhere it should, and with roughly the size it should. Of course (now this is a serious of course) we're in no position yet to make any quantitative statements about the properties of the Higgs. Indeed, measuring the couplings of the Higgs to matter will be the clue of the experimental particle physics program for the next 20 years. The more precisely we'll measure these couplings, the bigger chance there is to catch a glimpse of new physics. Still, it is getting more likely than ever that the Standard Model is the correct description of physics at the TeV energies. This is dubbed the nightmare scenario; in the first place a nightmare for particle theorists who become expendable, but in a 30 years perspective also a nightmare for the entire particle physics program. Before the start of the LHC I was giving the nightmare scenario a 50% chance. It's more than that now. Once the Higgs is formally discovered and it fits the Standard Model one... I'm getting cold shivers just thinking about it...

PS. Regardless, isn't it a beauty ;-)

Visual on Higgs

2011-12-13T16:12:00.023+01:00

Today CERN presented the latest Higgs search results. But, first things first. The live podcast of the seminar was stalling, choking, or breaking down completely. Well into the 21st century a huge international lab doing cutting edge science (and boasting of inventing the world wide web) is unable to provide a smooth transmission of its most important public presentation of the year. One may call it ironic, or repeat Didier Drogba's famous words after the 2009 Champions League semi-finals.

OK, after venting my anger I can talk some physics.

The main news, correctly rumored on blogs before, is that there is a significant excess of Higgs-like events corresponding to the Higgs mass of ~125 GeV. More precisely, the local significance of the ATLAS excess is 3.6σ, or 2.5σ sigma when the "look-elsewhere effect" in the 110-146 GeV mass range is taken into account. For the CMS the significance is somewhat smaller: 2.6/1.9 with/without the look-elsewhere effect. Separately, these excesses would be shrugged off; combined, they are very suggestive that we're seeing the real thing at last.
In ATLAS, only the H→γγ and H→ZZ*→2l+2l- channel have been updated with the full 2011 dataset. Those are the ones where statistics is limited due to the small Higgs branching fractions. However, in these final states the 4-momentum of the Higgs can be fully reconstructed to a good precision, offering a very good mass resolution of order 2 GeV. CMS updated all main channels, also those that do not provide a lot of mileage near 125 GeV at this point.
The excess is seen by both experiments and in each of these channels. The excess in H→γγ peaks around 124 GeV it CMS, and around 126 GeV in ATLAS, which I guess is perfectly consistent within resolution. In the 4-lepton channel, ATLAS has 3 events just below 125 GeV, while CMS has 2 events just above 125 GeV. On top of that there's the long-standing excess in the H→WW*→l+l-2ν channel, which however is not the driving force anymore. It's is precisely this overall consistency that makes the signal so tantalizing.
ATLAS observes slightly more events than expected from the Standard Model Higgs at that mass. In ATLAS the best fit to the Higgs cross section corresponds to roughly 1.5 the Standard Model value. Thus, one way or another, the 125 GeV thing is a fluke. It may have been good luck if these events are due to Higgs, or bad luck if they are due to other Standard Model backgrounds...
CMS now excludes Higgs down to 127 GeV. The ATLAS limits slightly worse around 130 GeV, but thanks to lucky background fluctuations they happened to exclude the low mass region between 112.7 and 115.5 GeV. The latter is a spectacular confirmation that what the ALEPH experiment saw back in 2001 was a genuine fundon (fundons are elementary particles produced in high-energy collider near the end of the budgetary cycle).

So much for now. The ATLAS and CMS combination notes are already out.

Tinker Taylor Soldier Higgs

2011-12-12T14:31:00.006+01:00

Tomorrow the ATLAS and CMS experiments will reveal new results of the Higgs search based on the full 2011 dataset. The date of the presentation was not chosen accidentally: December 13 is the anniversary of the capture of Saddam Hussein. Is this suggesting that tomorrow we'll see another notorious fugitive dragged out of its hole?

Two things are 100% certain because they appeared in an official statement from CERN:

Neither experiment will announce the discovery of the Higgs, in the sense of a signal with a significance of 5 sigma.
Neither experiment will exclude the Standard Model Higgs over the whole low-mass range.

The rest of the story can be to a good approximation reconstructed from blogs, facebook status updates, whispers in the corridors and coffee rooms, drafts mistakenly left on printers, microfilms smuggled in diplomatic mail, etc. The story that emerges can be summarized as follows:

The Standard Model Higgs boson is excluded down to approximately 130 GeV, but not below.
As already reported widely on blogs, both experiments have an excess of events consistent with the Higgs particle of mass around 125 GeV.
The excess is larger at ATLAS, where it is driven by the H→γγ channel, and supported by 3 events reconstructed in the H→ZZ*→4l channel at that mass. The combined significance is around 3 sigma, the precise number depending on statistical methods used, in particular on how one includes the look-elsewhere-effect.
CMS has a smaller excess at 125 GeV, mainly in the H→γγ channel. They have 3 events in H→4l as well, but they are oddly shifted to somewhat lower masses of order 119 GeV. All in all, the significance at 125 GeV in CMS is only around 2 sigma.
With some good faith, one could cherish other 2-sigmish bumps in the γγ channel, notably around 140 GeV. Those definitely cannot be the signal of the Standard Model Higgs, but could well be due to Higgs-like particles in various extensions of the Standard Model.

Stay tuned, in about 24 hours we'll know everything there is to know. More in-depth interpretations of the ATLAS and CMS findings depend on the quantitative details of the results, in particular, to what extent the signals in various channels are consistent between the 2 experiments and with the predictions based on the Standard Model Higgs. There is a good chance we're finally looking at the real thing, I'd say 50% based on the data alone and 80% adding our sincere convictions that Higgs must really be in that mass range. However, one should not forget that last summer the combination of ATLAS and CMS 1 fb-1 results was showing an excess corresponding to a ~145 GeV Higgs with the significance as large as 4 sigma, which was blown away when more data arrived.

You're of course welcome to fill in more details or paste an excerpt of the ATLAS or CMS draft into the comment section ;-)

Update on CP violation in charm

2011-12-02T22:38:00.005+01:00

This post came out a bit cryptic. Don't even start if you're not a huge fan of flavor physics.

There's been some new developments since 2 weeks ago when LHCb came out with the evidence of CP violation in the charm meson sector. Recall that LHCb studied the D-meson decay to π+π- and K+K- meson pairs. The observable of interest was the asymmetry of D- and anti-D-meson decay rates to these final states, A(π+π-) and A(K+K-). A non-zero value of any of these two asymmetries signals CP violation (particle and anti-particle decay rates to the same CP eigenstate being different). LHCb found that the difference ΔA = A(K+K-) - A(π+π-) is (-0.82±0.24)%, which means that, with large confidence, at least one of these asymmetries is non-zero.

The most pressing question is whether the LHCb result implies new physics? Experts emphatically agree that the answer is definitely maybe. From the existing literature one can learn that "...CP violation from new physics must be playing a role if an asymmetry is observed with present experimental sensitivities O(1%)", and "...observation of CP violation in the decay of D mesons will not necessarily be a signal of new physics...". Now, a paper from last week makes a reassessment of the Standard Model predictions and clarifies the LHCb result may or may not signal new physics.

The problem is that we are lacking reliable methods to compute processes involving D mesons. Normally, when dealing with heavy flavored mesons, one employs an effective theory where the heavy Standard Model and possibly new physics particles have been integrated out, leaving effective 4-quark interactions. This allows one to compute observables at the leading order in the expansion in powers of (1 GeV/m_quark), the former being the typical scale for QCD effects, and the latter the suppression scale of higher-dimensional operators. That is a decent expansion parameter for bottom quarks, but not so much for charm quarks. Now, ignoring that convergence issue and taking the leading order predictions at face value one arrives at the estimate ΔA∼0.1%, well below the value measured by LHCb. The new paper by Brod et al. attempts to estimate the contribution of the higher order operators to the asymmetry, using some guidance from other experimental data on D-mesons. For example, one finds that the branching fraction for D0 → K0 K0bar, which receives contributions only at next-to-leading order in 1/mc, is about five times smaller than the branching fraction of D0 → K+ K-, which received leading order contributions. That means that the respective amplitudes differ only by a factor of two, which in turn proves that the higher order 1/mc contributions can be significant. Taking that into account, the paper concludes that the Standard Model can account for |ΔA| as large as 0.4%, uncomfortably close to the LHCb measurement.

Another last week paper takes a less pessimistic approach. It simply assumes that the asymmetry measured by LHCb is dominated by new physics and attempts to understand constraints on the underlying model. In the language of effective theory, to explain the LHCb result one needs to include a ΔC =1 four-quark operator (with 1 charm and 3 light quarks, [sbar Γ c][ubar Γ s]). There are many possibilities differing by the structure of Lorentz and color indices that lead to the same observable asymmetry. The scale suppressing this higher dimensional operator should be of order 10 TeV to match to the LHCb result. This is actually a very small suppression. From non-observation of CP violation in D meson mixing we know that the scale suppressing generic ΔC=2 4-quark operators (with 2 charm and 2 light quarks) must be close to 10 000 TeV. On the other hand, once a ΔC=1 operator is present, ΔC=2 ones will necessarily be generated by loop corrections. This means that a random new physics model explaining the LHCb result will be in conflict with the data on D-meson mixing. However, the paper by Isidori et al. concludes that a subset of the ΔC=1 operators explaining the LHCb asymmetry is consistent with all other experimental data. In particular, the ΔC=1 operators involving only right-handed quarks are not excluded by the data on D-meson mixing and on direct CP violation in kaon decays (ε'/ε in the flavor jargon).

Finally, a small experimental update. CDF posted on arXiv a new paper that is similar to the earlier public note but contains one bonus track. The known result is the separate CP asymmetries D-meson decays to pions and kaons:

A(K+K-) = -0.24 ± 0.24 A(π+π-) = 0.22 ± 0.26

The bonus is that they did the subtraction and also present the difference of the asymmetries:

ΔA = -0.46 ± 0.33

which can be directly compared to the LHCb result. The CDF asymmetry difference is perfectly consistent with the LHCb one, and it's also consistent with zero. This is a hint that the true asymmetry difference is probably at a lower end of the range suggested by the LHCb measurement, and therefore closer to the Standard Model prediction. One more point for the Standard Model, but the game is not over yet.

New Higgs combination is out

2011-11-18T10:56:00.011+01:00

Here is a brief report about the long-awaited combination of the ATLAS and CMS Higgs search results using about 2 inverse femtobarn of data collected last summer. Overshadowed by faster-than-light neutrinos, that result was presented today at the HCP conference in Paris. It had been expected with as much thrill as results of parliamentary elections in the former Soviet Union. Indeed, in this fast-moving world 2 months ago is infinite past. Today particle physicists are rather busy rumoring the results based on the entire LHC data set of 5 inverse femtobarn. Moreover, the combined limits had not been difficult to guess, and a reasonable approximation of the official combination had been long available via viXra log.

Nevertheless, it's the chronicler's duty to report: the Standard Model Higgs is excluded at 95% confidence level for all masses between 141 GeV and 476 GeV.

Meanwhile, ATLAS and CMS have already had the first look at the full data set. Continuing the Soviet analogy, an uneasy rumor is starting among the working class and the lower-ranked party officials. Is the first secretary dead? Or on life support? Or, if he's all right, why he's not showing in public? We expect an official update for the 21st Congress of the Communist Party, sorry, the December CERN Council week. And wild speculations on Twitter well before that :-)

See the public note for more details about the combination.

LHCb has evidence of new physics! Maybe.

2011-11-14T19:52:00.028+01:00

It finally happened: we have the first official claim of new physics at the LHC. Amusingly, it comes not from ATLAS or CMS, but from LHCb, a smaller collaboration focused on studying processes with hadrons formed by b- and c-quarks. Physics of heavy quark flavors is a subject for botanists and, if I only could, I would never mention it on this blog. Indeed, a mere thought of the humongous number of b- and c-hadrons and of their possible decay chains gives me migraines. Moreover, all this physics is customarily wrapped up in a cryptic notation such that only the chosen few can decipher the message. Unfortunately, one cannot completely ignore flavor physics because it may be sensitive to new particles beyond the Standard Model, even very heavy ones. This is especially true for CP-violating observables because, compared to small Standard Model contributions, new physics contributions may easily stand out.

So, the news of the day is that LHCb observed direct CP violation in neutral D-meson decays. More precisely, using 0.58 fb-1 of data they measured the difference of time-integrated CP asymmetries of D→ π+π- and D→ K+K- decays. The result is

3.5 sigma away from the Standard Model prediction which is approximately zero!

Here is an explanation in a slightly more human language:

Much like b-quarks, c-quarks can form relatively long-lived mesons (quark-antiquark bound states) with lighter quarks. Since mesons containing a b-quark are called B-mesons, those containing a c-quark are, logically, called D-mesons. Among these are 2 electrically neutral mesons: D0 = charm + anti-up quark, and D0bar = anti-charm + up cbar-u quark. CP symmetry relates particles and anti-particles, in this case it relates D0 and D0bar. Note that D0 and D0bar mix, that is they can turn into one another; this is an important and experimentally established phenomenon which in general may be related to CP violation however in the present story it plays a lesser role .
D-mesons are produced at the LHC with a huge cross-section of a few milibarns. LHCb is especially well equipped to identify and study them. In particular, they can easily tell kaons from pions thanks to their Cherenkov sub-detector.
Here we are interested in D mesons decays to a CP invariant final state f+f- where f = π,K. Thus, the D0 → f+f- and D0bar → f+f- processes are related by a CP transformation, and we can define the CP asymmetry as
If CP was an exact symmetry of the universe, the asymmetries defined above would be zero: the decay probabilities into pions/kaons of D0 and D0bar would be the same. The Standard Model does violate CP, however its contributions are estimated to be very small in this case, as I explain in the following.
At the Tevatron and B-factories they measured separate measurements of the asymmetries A_CP(π+π-) and A_CP(K+K-) (obtaining results consistent with zero). LHCb quotes only the difference A_CP(K+K-) - A_CP(π+π-) because, at a proton-proton collider, the D0 and D0bar mesons are produced at a different rate. That introduces a spurious asymmetry at the detection level which, fortunately, cancels out in the difference. Besides, the mixing contribution to the asymmetry approximately cancels out in the difference as well. Thus, the observable measured by LHCb is sensitive to so-called direct CP violation (as opposed to indirect CP violation that proceeds via meson-antimeson mixing).
LHCb has collected 1.1 inverse femtobarn (fb-1) of data, 5 times less than ATLAS and CMS, because the LHCb detector cannot handle as large luminosity. The present analysis uses a half of the available data set. The error of the measurement is still dominated by statistics, so analyzing the full data set will shrink the error by at least Sqrt[2].
What does the good old Standard Model has to say about these asymmetries? First of all, any CP asymmetry has to arise from interference between 2 different amplitudes entering with different complex phases. In the Standard Model the 2 dominant amplitudes are:
#1: Tree-level weak decay amplitude. The pictured amplitude involves the CKM matrix elements V_us and V_cs, therefore it is suppressed by one power of Cabibbo angle, the parameter whose approximate value is 0.2.
#2: One-loop amplitude which, for reasons that should be kept secret from children, is called the penguin. Again it involves the CKM matrix elements V_us and V_cs, and also a loop suppression factor α_strong/π. However, as is well known, any CP violation in the Standard Model has to involve the 3rd generation quarks, in this case a virtual b-quark in the loop entering via V_cb and V_ub CKM matrix elements.
The corresponding D0 → π+π- amplitudes are of the same order of magnitude.
All in all, the direct CP asymmetry in the D0 → π+π- and D0 → K+K- is parametrically proportional to (α_strong/π) (Vcb*Vub)/(Vus*Vcs) which is suppressed by the 4-th power of the Cabibbo angle and a loop factor. This huge suppression factor leads to an estimate of the Standard Model contribution to the CP asymmetry at the level of 0.01-0.1%. On the other hand, LHCb finds a much larger magnitude of the asymmetry, of order 1%.
Is it obviously new physics? Experts are not sure because D-mesons are filthy bastards. With the masses around 2 GeV, they sit precisely in the no man's land between perturbative QCD (valid at energies >> GeV) and low-energy chiral perturbation theory (valid between 100 MeV and 1 GeV). For this reason, making precise Standard Model predictions in the D-meson sector is notoriously difficult. It might well be that the above estimates are too naive, for example the penguin diagram may be enhanced by non-calculable QCD effects by a much-larger-than-expected factor.
And what is it if it indeed is new physics beyond the Standard Model? This was definitely not the most expected place where theorists had expected new physics to show up. Currently there are almost no models on the market that predict CP violation in D0 decays without violating other constraints. I'm aware of one that uses squark-gluino loops to enhance the penguin, let me know about other examples. This gap will surely be filled in the coming weeks, and I will provide an update once new interesting examples are out.

So, is this new physics or the Standard Model? The LHCb result is definitely exciting, but the jury is still out. This time we need not only more data, but also a more inspired approach to understand the Standard Model predictions. Let's see what theorists will make of it. The only certain thing is that it's the first evidence of CP violation in the charm sector.For more insight see also Sean, Alexey, Tommaso. Thanks to Diego for enlightenment.

Double Dare

2011-11-11T17:23:00.016+01:00

(There's nothing like a little rant on a holiday morning)

Last Wednesday I noticed this press release from Double Chooz which announced "the observation of the disappearance of (anti-)neutrinos in the expected flux observed from the nuclear reactor" which implies "complementary and important evidence of oscillation also involving the third angle". Wow, I thought, they've nailed down theta13! But it turned out to be much more exciting than just another fundamental parameter. A more careful reading reveals that, based on the first 100 days of data, Double Chooz found sin^2(2 theta13) = 0.085 ± 0.051. Clearly something interesting is going on. To an untrained eye, the Double Chooz result is... consistent with zero; moreover it is similar, even if slightly less precise, to the null result from MINOS: sin^2(2 theta13) = 0.04 ± 0.04. However now in the 21st century one needs a more inspired approach to statistics...

To better understand what's going on, go back a few months. In June this year the T2K experiment also issued a press release about theta13, announcing "an indication that muon neutrinos are able to transform into electron neutrino". T2K is an experiment in Japan where a beam of muon neutrinos with GeV energies is produced in J-PARC and sent over a 300km tunnel ;-) to the SuperKamiokande detector. It is established that muon neutrinos can oscillate into tau neutrinos, the process being governed by the theta23 angle in the MNS neutrino mixing matrix whose value is close ot 45 degrees. If the angle theta13 in that same matrix is non-zero then the process ν_μ → ν_e is also allowed. For this reason, T2K was searching for an appearance of electron neutrino in the muon neutrino beam. The T2K announcement was based on the detection of 6 electron neutrino events, versus about 2 expected from background. At the time some of us wondered why they put such a spin on a merely 2.5 sigma excess, given that neutrino experiments had already produced many confusing results with similar or larger significance (LSND, MiniBoone, later OPERA). After all, neutrino beam experiments are plagued by difficult systematic uncertainties which are due to our incomplete understanding of the dirty hadronic physics involved in the beam production. Indeed, the subsequent results from MINOS turned out to disfavor the T2K central value of sin^2(2 theta13) of about 0.11.

In hindsight, the T2K press release was a pioneering step in data interpretation and the gauntlet was recently picked up Double Chooz. The latter experiment is targeting the transformation of anti-electron neutrinos into other type which, at short distances, is also controlled by the theta13 angle. More precisely, Double Chooz is looking for disappearance of MeV anti-electron neutrinos at a distance of 1 km away from the French nuclear reactor Chooz B where the antineutrinos are produced. They observe a small deficit of events in the energy range 2-5 MeV compared to the no-oscillation hypothesis, see the picture. While T2K was spinning a less-than-3-sigma excess, the Double Chooz press release made a further bold step and presented a less-than-2-sigma one as an evidence. There is still a long way to adapt the standards used in psychology and behavioral sciences. But, little by little, this approach could be applied to wider areas of physics, especially to high energy physics which suffers from dearth of discoveries. Just think of it: if we could call a 2 sigma excess an indication then every week the LHC could deliver an indication of new physics!

But, seriously... I also expect that the value of theta13 is non-zero and the experiments may be seeing the first hint of it. One argument is that global fits to the neutrino oscillation data point to sin^2(2 theta13) = 0.05 and 3 sigma away from zero. Besides, there is no compelling theoretical reason why theta13 should be zero (and if you believe in anarchy there is a reason to the contrary). The smoke should clear up in the next few years thanks to Double Chooz, Daya Bay, NOvA, and others. However the current experimental situation is far from being conclusive and the latest Double Chooz results did not change much in this respect, as can be seen in the fit to the right. I guess this could have been said without diminishing the importance of Double Chooz, and without treating the public as retarded...

See the web page of Double Chooz and this post on Quantum Diaries for more details.

Experimental success, theoretical debacle

2011-11-02T15:35:00.010+01:00

(This post is an attempt to catch up with October subjects that were being trendy when I was on leave from blogging, even though I suppose no one cares anymore)

This year's Nobel prizes were by all means exceptional. In blatant disregard of noble traditions, the prize in physics was given for a groundbreaking(!) and recent(!!) discovery without omitting any of the key contributors(!!!). Indeed, the discovery of accelerated expansion is one of the greatest triumphs of modern science. The measurements of supernovae brightness in the 90s and subsequent experiments have demonstrated that the universe is currently dominated by a form of energy characterized by negative pressure. In fact, this "dark energy" has the properties of the vacuum energy aka the cosmological constant, first introduced by Einstein for completely wrong reasons. In science, experimental progress usually brings better theoretical understanding. And that's another exceptional thing about the recent Nobel: almost 15 years after, the understanding of the cosmological constant in the context of particle physics models is as good as non-existent.

The cosmological constant problem has been haunting particle physicists for nearly a century now. We know for a fact that all forms of energy gravitate, including the energy contributed by quantum corrections. Thus, we know that diagrams with a graviton coupled to matter loops, like the one in the upper picture, yield a non-vanishing contribution to scattering amplitudes. On the other hand, the sum of very similar diagrams with graviton coupled to matter loops in vacuum must be nearly zero, otherwise the approximate Minkowski vacuum in which we live in would be destabilized. The contribution of the electron loop alone (the lower picture) is about 50 orders of magnitude larger than the experimental limit. On top of that, there should be classical contributions to the vacuum energy, for example from the QCD condensate and from the Higgs potential, which are also naturally tens of orders of magnitude larger than the limit.

The usual attitude in theory is that when something is predicted infinite one assumes it must be zero, and that was a good enough approach before 1998. The discovery of accelerated expansion was a game-changer, because it experimentally proved that the vacuum energy is real and affects the cosmological evolution, therefore the problem can no longer be swiped under the carpet. In fact, the problems is now double. Not only we need to understand why the cosmological constant takes a highly unnatural value from the point of view of the effective low-energy theory (the old cosmological constant problem), but we need to understand why it is of the same order as the matter energy density today (the coincidence problem).

Neither the first nor the second problem has found a satisfactory solution to date. Not for a lack of trying. People have attacked the problem via IR and/or UV modifications of gravity, quintessence fields, self-tuning or attractor solutions, fancy brane configurations in extra dimensions, elephants standing on turtles, space-time wormholes, etc, see also the comment section for crazier examples. In vain, all these solutions either rely on theoretically uncontrollable assumptions, or they just shift the problem somewhere else. The situation remains so dramatic that there are 2 only solutions that are technically correct:

The anthropic principle: the cosmological constant is an environmental quantity that takes different values in different patches of the universe, however more-or-less intelligent observers can see only those tiny patches where it is unnaturally small.
The misanthropic principle: the cosmological constant is being adjusted manually by seven invisible dwarfs wearing red hats.

Both of these theories have a comparable predictive power. In the first case we currently have no way to know the fundamental theory that sets the statistical distribution of the cosmological constant. In the second case we don't know what the little bastards are really up to.

Maybe theory needs another clue that may be provide by one of the future experiments. The Planck satellite will publish an update on cosmological parameters in 2013, although the rumor is that there won't be any revolution. In the asymptotic future there is ESA's Euclid satellite who will precisely measure the distribution of dark matter and dark energy in the universe. Will I live to see the day when the problem is solved? My bet is that no, but I'd love to proven wrong...

For the best summary of the cc problem read Section 1 of Polchinki's review.

What if they don't find the Higgs?

2011-10-27T20:05:00.010+01:00

(...No I didn't cut my wrists after the Tevatron shutdown, contrary to what you might have concluded from my blogging history...)

So, the 2011 run of the LHC is coming to a close, I mean the interesting part ;-). A 5 inverse femtobarn stash of data has been collected by each ATLAS and CMS. These data will by fully analyzed and scrutinized by the late winter 2012, while rumors should start popping up on blogs before the end of this year. One thing that is already clear is that new physics did not jump in our faces, which is hardly a surprise. And neither did the Higgs boson, which is more intriguing. Contrary to what I expected, the 2011 data may not yield a conclusive statement about the Higgs: neither a clear cut discovery nor excluding the entire low mass range appears likely at this point. We can now at least entertain the option, which as recently as last summer was unthinkable, that the LHC will not find the Higgs particle with the properties predicted by the Standard Model. What then?

First of all, it will be fun to watch the CERN management explaining the public that *not* discovering the Higgs is a success. For theorists, on the other hand, the best of all worlds will have been granted. In fact, we already have a deck of cards to play for that occasion, each very interesting as each pointing to exciting new physics within our reach. Here are the 3 main broad scenarios (not mutually exclusive):

Higgs exists but has a smaller production cross section.
In the Standard Model the Higgs is produced mostly in gluon fusion, via a loop diagram with top quarks. One can easily imagine new particles meddling into Higgs production via a similar loop process; all they need is a color charge and a significant coupling to the Higgs. Thus, in every major new physics scenario modifying the Higgs production rate is possible without stretching the parameters too much. One interesting case is the composite Higgs, where the Higgs cross section is almost always suppressed, typically down to 70-90% of the Standard Model value. For experimentalists this is the simplest scenario, all they need to do is sit and wait a bit longer, and the Higgs will eventually show up. The matter should be sorted out after the 2012 data are analyze.
Higgs exists but has non-standard decays.
For a low mass Higgs, around 120 GeV, the main discovery channel is the decay into 2 photons. Again, this is a loop process in the Standard Model so it's very easy for new physics to modify the branching fraction for that decay. As in the previous case, one may just sit and wait for the Higgs to eventually show up. However Higgs decays can be easily modified in a far more dramatic fashion than the production rate. For example, Higgs may be invisible, that is decaying into very weakly interacting particles whose only signature is the unbalanced momentum in the event. Or Higgs may dominantly decay into multiparticle final states (some popular model predict decays to 4 tau leptons or 4 b-quarks) and we'll never see the bastard in the diphoton channel. That would be a very interesting scenario not only for theorists but also experimentalist, as it would require clever new methods to spot the Higgs on top of the QCD background.
There is no Higgs.
That would mean that the mechanism of electroweak symmetry breaking is inherently strongly coupled, somewhat resembling breaking of the chiral symmetry in QCD. This is the most challenging scenario for theorists and experimentalists, and the one that may require a lot of patience. In the optimistic case, the 14 TeV LHC run we will spot a number of resonances analogous to QCD mesons and little by little we'll understand the structure of the underlying gauge theory. But these resonances may well be too heavy or too wide to be efficiently studied at the LHC. Ultimately, we may need to probe the properties of the scattering amplitudes of W and Z bosons where, according to theory, these strong interactions must leave an imprint. The problem is that such a measurement is very non-trivial in the dirty LHC environment (the SSC or a linear collider would be a different story), so we may need some new theoretical or experimental ideas to make the progress. It's probably too early to bet large amounts on this scenario (which is currently disfavored by electroweak precision data) but if no hint of the Higgs is seen by the end of 2012 that will become the most promising direction.

In summary, discovering the Higgs would be a big news, a huge achievement of the whole community, one small step for mankind, et cetera. But not discovering it would be more exciting by a lot, a lot, a lot. Of course, assuming that eventually we will find something ;-)

Live from Fermilab: Chronicle of a Death Foretold

2011-09-30T01:55:00.081+01:00

2:38 pm: It's over. The heart stopped 2:38 pm, the last store number was 9158. Good night.
2:37 pm: Helen Edwards all too eagerly pressed the big red button to dump the beam. Soon she will press the big green button to ramp down.
2:35 pm: Stop Helen, I'm afraid
2:35 pm: The heart is still beating but the brain is dead: Tevatron no longer records the data.
2:34 pm: ...though I must say that the CDF show was much more entertaining.
2:32 pm: D0 run terminated. They're ramping down.
2:29 pm: Somehow the whole ceremony reminds me of this scene.

2:28 pm: Time for D0, the better of the 2 Tevatron experiments ;-) Bill Lee from the D0 control room.
2:25 pm: The CDF run has been terminated, 2 million events collected. CDF no longer takes data.
2:22 pm: There is now a story of chickenpox children sacrificed at the altar of science. You don't want to know how it ends.
2:16 pm: Ben Kilminster live from the CDF control room says that back in 1985 there was only one monitor there. There was also no blogs, Twitter or Facebook. Clearly there is some progress...
2:15 pm: Soon the detectors will start shutting down. They don't to watch it...
2:10 pm: Tour of the control room. Looks like space movies from the 70s with lots of color lights blinking.
2:o4 pm: It started. Booooo. Pier Oddone, the director of Fermilab, speaking.
2:o1 pm: Nothing's happening yet. The stream shows photos of serious faces staring at monitors or parts of the accelerator.
1:57 pm: I wonder what will happen to the buffaloes... Will they all be slaughtered and served at the funeral party in the Wilson Hall autrium?
1:50 pm: Except for the top quark, is the Tevatron going to be remember for anything? In the coming years their measurement of the top quark and the W boson mass will remain the most precise one - the LHC will have to struggle hard to beat it. Moreover, a number of measurements - especially various production asymmetries - cannot be repeated at the LHC.
1:45 pm: The Tevatron will die today but the ghost will linger on a bit longer. Physics analyses based on the full dataset are expected only in about 5 months, for the winter 2012 Moriond conference. After that the trickle will be slowing down, but papers and analyses should will be coming up for several more years.
1:40 pm: Streaming of the execution will begin in about 5 minutes.
1:30 pm: Memorial photo of the D0 collaboration in the pit. Not much time left...
1:10 pm: Dismantling of the Tevatron will begin in about a week, shutdown, as soon as the superconducting magnets are warmed up to the room temperature. The CDF detector will also be shut down today, while D0 will be operating for 3 more months to get a sample of cosmic events for calibration purposes. I'm not aware of any plans of reusing parts of these detectors for other experiments.
12:50 pm: With the shutdown of the Tevatron, Fermilab is losing its dearest child and the place at the forefront of high-energy physics, but for a while it will remain an important laboratory running smaller scale experiments. The dark matter detector COUPP, or the neutrino experiment MINOS will be producing important results that may even make it to blogs ;-) Construction of Mu2e, an interesting experiment to study lepton flavor violation, will begin in 2013. In the long run, however, the future of Fermilab looks bleak. Most likely it will share the fate of other once great US labs, like BNL or SLAC: sliding slowly into insignificance.
12:10 pm: One more statistically significant departure from the Standard Model was reported by the Tevatron: the dijet mass bump in W+2j events at CDF. Unfortunately, the effect was not confirmed by D0. It's not clear if this will be sorted out anytime soon...

12:10 pm: The shutdown of the Tevatron should be viewed as a part of the bigger program of shutting down fundamental research in the US. It makes sense: since manufacturing could be outsourced to China, no reason why research could not.
12:05 pm: Here you can see the current status of the accelerator. The luminosity is low but the old chap should make it all the way to the end.
11:45 am: Wonder how the execution will be carried out? In the state of Illinois they do it as follows:

...Helen Edwards, who was the lead scientist for the construction of the Tevatron in the 1980s, will terminate the final store in the Tevatron by pressing a button that will activate a set of magnets that will steer the beam into the metal target. Edwards will then push a second button to power off the magnets that have been guiding beams through the Tevatron ring for 28 years...

I think there should be 3 people, each pressing a button, only one of which is actually connected to the kicker...
11:40 am: It's a beautiful autumn day here in Fermilab today, unusually beautiful. Nature refuses to mourn.
11:05 am: The Tevatron has 3-4 more hours to live.
11:00 am: Except for the top asymmetry, another Tevatron's measurement returned a result grossly inconsistent with the Standard Model, namely, the dimuon charge asymmetry at D0. Although the interpretation of this result in terms of anomalous CP violation in the B-meson sector has been put to doubt by recent LHCb measurements of related processes, formally the D0 result still stands 10:45 am: The gravestone is ready even before the actual death:

10:15 am: So, Tevatron Run I got the top quark. Run II, which started in 2001, had 2 major goals: find the Higgs and find new physics. From this perspective one must admit that, \begin{evenif} insert here how great job was done \end{evenif}, Run II was a disappointment.
9:50 am: Except for the top quark, what were the most important findings of the Tevatron? See the list at Tommaso's blog.
9:30am: Tevatron's observation of the anomalous top-antitop forward-backward asymmetry is currently the strongest hint that there may be new physics. The fact it is the strongest is not really encouraging ;-)
9:15am: A bit of nostalgia: a page in Particle Data Group from 1996
9:00am: The LHC is leading the game in most of the Higgs search channels, but for the moment the Tevatron has a far better sensitivity to a light Higgs boson decaying to a pair of b-quarks. Interestingly, they see no excess in this channel (the excess in the combination comes mostly from the H to WW channel), even though they should if the Higgs is there...
8:40am: The eulogies have begun. For the next 2 hours I'll listen to the summary of the most important results obtained by the D0 collaboration.
8:30am: They're still accumulating antiprotons; a sort of life support in case the Tevatron trips before the scheduled time.
8:10am: The last store of protons and antiprotons is circulating in the ring since last evening. Current luminosity: 100 ub/sec, more than 3 times below the peak luminosity. Clearly, the Tevatron is already flatlining.
8:00am: The Tevatron will go down in history as the place where back in 1995 they discovered the top quark - probably the heaviest elementary particle.
7:50am: Tevatron's first beam was in 1983 so he's dying at 28. One year more than Janis Joplin, Jimmy Hendrix, Jim Morrison, Kurt Cobain and Amy Winehouse. What's similar is that death is coming is when the career is already on the decline.
7:45am: I'm wide awake, it's morning in Fermilab. Putting on my best suit and setting off to the funeral. In less than 7 hours the Tevatron will be no more...

The Phantom of OPERA

2011-09-23T23:11:00.011+01:00

Those working in science are accustomed to receiving emails starting with "dear sir/madam, please look at the attached file where I'm proving einstein theory wrong". This time it's a tad more serious because the message comes from a genuine scientific collaboration... As everyone knows by now, the OPERA collaboration announced that muon neutrinos produced at CERN arrive to a detector 700 kilometers away in Gran Sasso about 60 nanoseconds earlier than expected if they traveled at the speed of light (incidentally, trains traveling the same route arrive always late). The paper is available on arXiv, and the video from the CERN seminar is here.

OPERA is an experiment who has had some bad luck in the past. Its original goal was to study neutrino oscillations by detecting the appearance of tau neutrinos in a beam of muon neutrinos. However due to construction delays their results arrive too late to have any impact on measuring the neutrino masses and mixing; other experiments have in the meantime achieved a much better sensitivity to to these parameters. Moreover, the "atmospheric" neutrino mass difference, which enters the probability of a muon neutrino oscillating into a tau one, turned out to be at the lower end of the window allowed when OPERA was being planned. As a consequence, a fairly small number of oscillation events is predicted to occur on the way to Italy, leading to the expectation of about 1-2 tau events to be recorded during experiment's lifetime (they were lucky to already get 1). However they will not walk off the stage quietly. What was meant to be a little side analysis returned the result that neutrinos travel faster than light, confounding the physics community and wreaking havoc in the mainstream media.

I'm not very original in thinking that the result is almost certainly wrong. The main experimental reason, already discussed on blogs, is the observation of neutrinos from the supernova SN1987A. Back in 1987, three different experiments detected a burst of neutrinos, all arriving within 15 seconds and 2-3 hours before the visible light (which agrees with models of supernova explosion). On the other hand, if neutrinos traveled as fast as OPERA claims, they should have arrived years earlier. Note that the argument that OPERA is dealing with muon neutrinos while supernovae produce electron ones is not valid: electron neutrinos have enough time to oscillate to other flavors on the way from the Large Magellanic Clouds. One way to reconcile OPERA with SN1987A would be to invoke a strong energy dependence of the neutrino speed (it should be steeper than Energy^2), since the detected supernova neutrinos are in the 5-40 MeV range, while the energy of the CERN-to-Gran-Sasso beam is 20 GeV on average. However OPERA does not observe any significant energy dependence of the neutrino speed, so that is an unlikely explanation either.

From the point of view of theory the chances that the OPERA result being true are no better as there is no sensible model of tachyonic neutrinos. At the same time, we've been observing neutrinos in numerous experiments and in various different settings, for example in beta decay, from terrestrial nuclear reactors, from the Sun, in colliders as missing energy, etc. Each time they seem to behave like ordinary fermions obeying all rules of the local Lorentz invariant quantum field theory.

We should weigh this evidence against the analysis of OPERA which does not appear rock solid. Recall that OPERA was conceived to observe tau neutrino appearance, not to measure the neutrino speed, and indeed there are certain aspects of the experimental set-up that call for caution. The most worrying is the fact that OPERA has no way to know the precise production time of a neutrino it detects, as it could be produced anytime during a 10 microsecond long proton pulse that creates the neutrinos at CERN. To go around this problem they need a statistical approach. Namely, they measure the time delay of the neutrino arrival in Gran Sasso with respect to the start of the proton pulse at CERN. Then they fit the time distribution to the templates based on the measured shape of the proton pulse, assuming various hypotheses about the neutrino travel time. In this manner they find that the best fit is for the travel time is 60 nanoseconds smaller than what one would expect if the neutrinos traveled at the speed of light. However, one could easily imagine that the systematic errors of this procedure have been underestimated, for example, the shape of the rise and the fall-off of the proton pulse have been inaccurately measured. OPERA does a very good job arguing that the distance from CERN to Gran Sasso can be determined to 20 cm precision, or that synchronizing the clocks in these two labs is possible to 1 nanosecond precision, but the systematic uncertainties on the shape of the proton pulse are not carefully addressed (and, during the seminar at CERN, the questions concerning this issue were the ones that confounded the speaker the most).

So what's next? Fortunately OPERA appears to be open for discussion and scrutiny, thus the issue of systematic uncertainties should be resolved in the near future. Simultaneously, the MINOS collaboration should be able to repeat the measurement with similar if no better precision, and I'm sure they're already sharpening their screwdrivers. In the longer timescale, OPERA could try to optimize the experimental setting for the velocity measurement. For example, they might install a near detector on the CERN site (where there should be no effect if the current observation is due to neutrinos traveling faster than light, or there should be a similar effect if there is an unaccounted for systematic error in the production time). Or they could use shorter proton pulses, so that the neutrino production time can be determined without statistical gymnastics (it appears feasible - the LHC currently works with 5 ns bunches). I bet, my private level of confidence being 6 sigma, that the future checks will demonstrate that neutrinos are not superluminal... in the end the character from the original book turned out to be 100% human. But, of course, the ultimate verdict belongs not to our preconceptions but to experiment.

Summer's almost gone

2011-09-14T18:27:00.008+01:00

The season of the year known as summer conferences is over now. What will follow is probably two quiet months when particle physicists make provisions for winter. The LHC has recently restarted at a higher-than-ever luminosity in a bid to double the 2011 data set. Interesting new results are therefore not expected before November when the current run will end. All in all, this is a perfect moment for a post of the sort d'où venons nous blabla. Here's a summary of the most important events and cultural phenomenons of the past summer.

Higgs Chase
It was like one of these action movies where a fugitive surrounded by hundreds of police with guns and helicopters manages to escape disguised as a hostage. The odds of discovering Higgs this summer were significant, but the bugger chose its parameters so as to maximize the difficulty of being found. Nevermind, next time. A plot from this talk of Bill Murray, which is just an extrapolation of the current search sensitivity to larger data sets, shows that we're very close now to ultimate answers. With 5 inverse femtobarns of data CMS alone should be able to get a 3-4 sigma exclusion even in the worst possible case of a 115 GeV Higgs. ATLAS looks worse on that plot because their pT threshold for detecting leptons is set higher than that of CMS. This fact does not matter too much for a moderately heavy Higgs, but for a light one it punishes them (it's then easier to miss the H → WW → 2l 2ν decay which would produce rather soft leptons). If this can be improved we'll get an even better reach after combination of ATLAS and CMS data, close to the CMSx2 line. So by the end of the year we should know much more than today: 5 sigma discovery probably no, 3 sigma hint or exclusion probably yes.
Apocryphal Combinations
That was definitely the hit of the summer, on par with James Blunt. Although experts warn about their quality, although CERN authorities threaten with corporal punishment to anyone caught watching one, bootleg combinations of ATLAS and CMS Higgs results are thriving on blogs and even in LHC experimenters' talks. Coming next are apocryphal data analyses and, who knows, maybe apocryphal colliders.
Conference Revival
During the past decade particle physics conferences have been a sad and boring display. With the LHC running at full speed things have changed a lot. At least in theory, a spectacular discovery may now occur anytime which creates big expectations and excitement around the major conferences. Of course, in the 21st century there is no logical reason to present results at conferences. Experimental results could be presented for example once they're ready, and the presentations more efficiently via internet. Nevertheless, one should not neglect the important convivial aspects of conferences which play a similar role to maypole festivities in pagan societies. Not to mention that the conference deadlines provides an efficient whip for PhD students to finish the analysis in time.
SUSY Scorned
They say you don't kick a man when he's down. Another approach, more popular where I come from, is that it is precisely the best moment as he has limited options to retaliate. I'm somewhat torn between these two approaches. On one hand, watching someone once noble being tarred-and-feathered is always delightful. On the other hand, I sympathize with the view that the backlash against SUSY that is currently unfolding in the mainstream media has no logical grounds. Before the LHC one had to believe in a hundred of new particles just behind the corner who conspire not to break any of the accidental/approximate symmetries of the Standard Model such as the baryon, flavor, or CP symmetry, and in addition their contributions to the electroweak scale accidentally cancel at the 1% level. Now one has to believe in a hundred of new particles just behind the corner who conspire not to break any of the accidental symmetries of the Standard Model, whose contributions to the electroweak scale accidentally cancel at the 1% level, and who do not produce spectacular signals in the early LHC data. In this sense, the summer 2011 LHC results only infinitesimally changed the situation of supersymmetry.
Nihil Novi
Unfortunately, it's not only SUSY that is missing; new physics in whatever form obstinately refuses to show up. Not even a rumor these days... Especially disappointing is that the LHC, unlike the Tevatron, does not see any non-standard effects in top physics. Before, I was estimating the chances of LHC discovering new physics at about 50%. Now it is closer to 33%. The moment we discover the Higgs looking roughly like the Standard Model one, these chances will drop face down...
Sunset of Tevatron
This summer we have watched the Tevatron falling helter-skelter into obsolete. In most of the analyses the sensitivity of the LHC is now far superior. There are some notable exceptions though, such as the top and W mass precision measurement, light Higgs decays to b-quarks, and the forward-backward asymmetry of the top quark production. For these and other reasons, even though Tevatron's life-support will be switched off at the end of this month, the ghost will haunt us a little longer.

To finish with important events of this summer, Resonaances now has a Twitter account. Well, these days every celebrity has one ;-) Disappointingly, you won't learn from it what I had for breakfast, or about my views on the political tensions in southern Uzbekistan. It is going to be a low-traffic twitter limited to announcing new posts on Resonaances, pointing to interesting papers, blog posts and articles elsewhere, and spreading lesser rumors and gossips.

Cresstfallen

2011-09-07T13:21:00.009+01:00

Until yesterday CRESST was a sort of a legend: everybody heard of them but nobody ever saw them. That is to say, the CRESST excess has been informally discussed for a long time. Moreover, the events from one of the modules displaying an excess of events in the oxygen band have been shown at conferences since more than a year. However we were in the dark about the significance of the excess, backgrounds and systematic effects. Now CRESST has finally come out with a paper that spells out the excess and provides interesting details.

CRESST is in a way the fanciest of all dark matter experiments. Located in the Gran Sasso underground laboratory, it uses the target of CaWO4 crystals cooled down to 10 miliKelvins. When a particle scatters inside the target the deposited energy is converted into phonons and scintillation light, both of which can be detected. The light-to-phonon ratio helps discriminating the dark matter signal from backgrounds, for example electrons and photons produce mostly light. Furthermore, that ratio depends on the atom of the crystal molecule on which the scattering occurred: it is largest for oxygen, intermediate for calcium, and smallest for tungsten. This leads to characteristic bands in the light yield vs. recoil energy plane that you can see in the plot above showing events from one of the eight CRESST modules used in this analysis. These bands provide another handle on the signal, as heavy dark matter would show up mostly via scattering on tungsten, while the light one would pop up in the oxygen band.

At the same time CRESST is paying a price for their innovative technology, as they have to deal with incalculable and sometimes unexpected backgrounds. Apart from the usual neutron background and the leakage of e/γ events into the signal region they had to face α particles and Pb atoms emitted from the clamps holding the crystals, not to mention the exhaust fumes from the nearby DAMA detector. Some of these backgrounds will be reduced in future runs, but for the moment CRESST needs to estimate their contribution in the signal region using sideband analysis. Having done so, CRESST finds that a fraction (slightly less than a half) of the 67 events in the signal region cannot be understood in terms of the known backgrounds. Therefore they study the likelihood of the background plus dark matter signal hypothesis assuming vanilla elastic scattering of dark matter on the target. Here is their result for the preferred mass and cross section of the dark matter particle:
The likelihood function has 2 minima corresponding to 4.7 and 4.2 sigma rejection of the background-only hypothesis. We can safely forget about the deeper one: for these parameters Xenon100, CDMS and Edelweiss would see an elephant in their data. The shallower minimum, where the preferred dark matter mass is 9-15 GeV, also seems excluded by orders of magnitude. This one however lies in the tantalizing proximity to the CoGeNT and DAMA preferred region; actually the mass region (though not the cross section) perfectly agrees with the DAMA low-mass region. Some argue that CDMS and Xenon collaboration grossly overestimate their sensitivity near the threshold. This may be imagined in the case of 5-7 GeV dark matter, in which case combining experimental and astrophysical uncertainties with some good will and the presumption of innocence one can try to argue that the CoGeNT signal is marginally consistent with the Xenon and CDMS exclusion limits. On the other hand, 10 GeV dark matter would produce observable signals further away from the threshold of these 2 experiments, and it's unlikely it could escape their attention. Therefore, given CRESST is facing pesky backgrounds very similar to the suspected signal (both in spectral shape and the order of magnitude), the hypothesis of unknown and/or underestimated backgrounds faking the signal is currently the most probable one.

Summarizing, the new CRESST results are welcome and illuminating but they do not change significantly the landscape of dark matter searches. Clearly, experiment is closing in on IDM; what is not clear is whether that stands for Inelastic or Italian Dark Matter ;-)

See also Lubos, Matt, and again Matt.

LHCb says: no Bs anomaly

2011-08-27T13:48:00.012+01:00

The LHC is dominated by 2 monstrous collaborations of ATLAS and CMS. The LHCb experiment is their shy and bullied little brother whose focus is on B-physics. Nevertheless, there is a reason to pay more than usual attention to LHCb results because of several B-physics related anomalies coming from other experiments (see this talk for a wrap-up). The most exciting of those is the DZero measurement of the di-muon charge asymmetry which displays a 4 sigma deviation from the Standard Model prediction and points to an anomalously large CP violating phase of Bs-Bsbar meson mixing. The LHCb experiment is now reaching the level of precision that allows them to test these claims and, provided they're real, get a clear evidence of physics beyond the Standard Model. If this were a Hollywood movie the underdog would come up with a spectacular discovery winning everyone's respect and cheerleader's heart. But life is more like a Ken Loach movie...

Today at Lepton-Photon'11 LHCb presented a new analysis of CP violation in Bs meson decays to J/Ψ and ϕ (J/Ψ is a spin-1 bound state of c-cbar identified by its decay to μ+μ-, and ϕ is a spin-1 s-sbar bound state whose leading decay is to K+K-). This decay process is sensitive to the Bs-Bsbar mixing phase via the interference of the decay amplitudes with and without mixing. In this case the presence of CP violation does not have a spectacular consequence (like e.g. for the di-muon charge asymmetry), it just affects in a complicated way the distribution of the decay products. The LHCb detector can pinpoint the original flavor of the Bs meson (whether Bs or Bsbar), the time between production and decay, and the angular distribution of the muons and kaons from this decay. Using all this information they can simultaneously fit the mixing phase φs and the width difference ΔΓ between the two Bs meson mass eigenstates, other relevant parameters like the mass eigenvalues being well measured in previous experiments. Non-zero φs signals CP violation. The Standard Model predicts a small effect here, φs = -0.04, which is below the current sensitivity but new physics could easily produce a much larger phase. The result that LHCb finds looks like that
The phase φs is found to be 0.13 ± 0.2, in a good agreement with the Standard Model prediction. Furthermore, LHCb analyzed different, less frequent Bs decays to J/Ψ f0 (the f0 meson has the same quark content as ϕ but it has 0 spin and decays dominantly to π+π-) which provides another independent determination of φs and ΔΓ. Combining it with the previous one, the experimental error on φs does not change much but the central value is shifted to 0.03.

This result is extremely disappointing. Not only LHCb failed to see any trace of new physics, but they also put a big question mark on the D0 observation of the anomalous di-muon charge asymmetry. Indeed, as can be seen from the plot on the right, the latter result could be explained by a negative phase φs of order -0.7, which is now strongly disfavored. In the present situation the most likely hypothesis is that the DZero result is wrong, although theorists will certainly construct models where both results can be made compatible. All in all, it was another disconcerting day for our hopes of finding new physics at the LHC. On the positive side, we won't have to learn B-physics after all ;-)

Higgs won't come out of the closet, part II

2011-08-23T02:55:00.015+01:00

After a short summer break we're back to Higgs hunting. The LHC continues to exceed all expectations with regard to the machine performance as it continues to disappoint (or to test our patience, if you prefer) with regard to discoveries. The latest Higgs search results based on about 2 inverse femtobarns of data were presented by ATLAS and CMS yesterday at the Lepton-Photon conference in Mumbai (though properly it should be called Lepton-Photon-Jet-and-Missing-Energy). The last status update: still no Higgs in sight.

Nothing new at first sight, so what's new?

Within the framework of Standard Model the Higgs boson is excluded by at least one experiment in the mass range 145-466 GeV, except for a small 288-296 GeV window that probably would also be excluded if ATLAS and CMS results were combined. Furthermore, the Standard Model Higgs heavier than 466 GeV is by far excluded by precision electroweak observables, mostly by the precise measurement of the W and Z boson masses to which Higgs contributes at the quantum level. This leaves 115-145 GeV as the most likely hiding place. That range shrinked only by a few GeV compared to the limits presented at EPS a month ago.
CMS updated several Higgs search channels with 1.5-1.7 fb-1 of data. ATLAS, on the other hand, updated only the 2 channels which provide most of the steam : H→WW→2l2ν and H→2Z→4l, although throwing in a bit more data than CMS. That is because ATLAS is more dependent on European workforce which in August retreats en masse to the seaside.
After the EPS conference there was a reasonable hope that an evidence for the Higgs could emerge this summer. The previous LHC results were suggestive of a 140-ish GeV Higgs boson producing a broad excess in the H→WW→2l2ν channel. Now it seems that a 140 GeV Higgs is not preferred by the latest data, even if it's not formally excluded: as Tommaso explains in these two posts, if the Higgs has indeed 140 GeV we would expect a larger excess by now. A lighter Higgs, 115-130 GeV, remains perfectly consistent with the data, in the sense that we would not expect to see it just yet.
The sample of the "golden-channel" final state with 2 Z bosons decaying to 2 leptons each is growing in size but nothing glitters here. This channel is the leading one for the heavy Higgs, and it retains some sensitivity for intermediate masses above 140 GeV. Unfortunately, the shape of the ZZ invariant mass spectrum that emerges has no significant bumps and nicely follows the background continuum. The di-photon sample, whose sensitivity is approaching the Standard Model cross section for a light Higgs, shows no interesting bumps either (the plot below).
It is somewhat surprising that the LHC didn't show the combination of 1fb-1 ATLAS and CMS data, contrary to what they promised. Probably they decided it would be confusing as the excess seen in the earlier data is not being confirmed by the newer data. Another hypothesis is that they didn't show it because the plot turned out identical to the one on viXra log ;-)
One should not forget that the LHC limits refer to the Standard Model Higgs. Beyond the Standard Model the Higgs may have a reduced cross section, larger width, invisible or more pesky decays, and so on. Any of these modifications may invalidate the Standard Model limits and make the search more challenging. For the moment the standard Higgs is the priority but we'll think more seriously about the alternatives in case no evidence is seen in 5fb-1. Furthermore, going beyond the Standard Model, a very heavy Higgs above 450 GeV becomes formally allowed provided some other particles mess up into our precision observables.
Finally, one can't help but notice that the Higgs, if it exists in the Standard-Model-like avatar, chose its own mass so as to maximize the difficulty of discovering it. If it's a god particle it's Loki rather than Thor.

The next major Higgs update will probably wait until this year's LHC run is completed, that is until November. Is there anything else we should expect during Lepton-Photon? According to the Bollywood rules of the genre there must be a happy ending with everybody dancing in the last scene. Actually, there is a persistent rumor among theorists that LHCb, whose presentation is scheduled for Saturday, is sitting on an interesting result. Is this true and, if so, will they share it in Mumbai? Experience shows you should not trust theorist-driven rumors but, regardless, it may be worth to wake up early on Saturday :-)

D0: top forward-backward asymmetry continues to intrigue

2011-07-26T00:46:00.009+01:00

While the LHC has been depressingly confirming the predictions of the Standard Model, the good old Tevatron remains the only light in the tunnel. The only 2 lights actually: almost 4 sigma anomaly of the dimuon charge symmetry measured by D0, and over 3 sigma anomaly of the top quark forward-backward asymmetry at high t-tbar invariant mass (yikes) measured by CDF. Future will tell whether these lights should be interpreted as the way out or as the oncoming train. As for the present, the D0 collaboration just provided an important update concerning the forward-backward asymmetry that puts the CDF result in a slightly different... light.

For the latest update D0 used 5.4fb−1 and focused on semi-leptonic top decays. The idea of the measurement is simple: one picks the top decay products, reconstructs the original top and anti-top momenta, and check for an excess of top over antitop quarks moving forward (that is along the proton beam) in the t-tbar rest frame. The Standard Model predicts such an excess should be very small, of order 5%. Instead, after unfolding detector effects from the measured asymmetry, D0 finds the "unfolded" or "production" asymmetry to be 19.6 ± 6.5 %. This kicks in very nicely with the analogous CDF result of 15.8 ± 7.4 % (or 20% when combined with the asymmetry in the dilepton channel). Both results are about 2 sigma away from the Standard Model and both point in the same direction, which is intriguing and almost exciting.

However, not everything agrees perfectly between D0 and CDF. Most importantly, D0 does not see any significant dependence of the asymmetry on the t-tbar invariant mass. On the other hand, CDF sees a dramatic dependence: it was actually the abnormally high asymmetry in the mtt > 450 GeV bin that allowed them to claim a 3-sigma anomaly at the beginning of this year. The situation is thus a bit volatile: the good matching of the inclusive asymmetry between the two experiments is obtained after integrating over discrepant results in the low and high mtt bins.

D0 shares one more important result which I find more exciting. D0 measured the leptonic asymmetry of the leptons originating from top quark decays. The observable is defined as an excess of positive charge leptons moving forward + negative charge leptons moving backward over negative (positive) leptons moving forward (backward). For experimentalists it is more user-friendly than the top asymmetry : it is defined in the laboratory frame and one can avoid tedious and uncertain reconstruction of the momenta of the top quarks. From the theoretical point of view, the lepton asymmetry is tightly related to the top forward-backward asymmetry but not identical. It is related, because the direction of the lepton is clearly correlated with the direction of the mother top or antitop. It is not identical, because that direction is also correlated with the polarization of the mother top. In fact, if the top quark is polarized along some axis, for example in the direction of its motion, the lepton prefers to fly along that direction. See for example this paper for more details. All in all, D0 finds this leptonic asymmetry to be 15.2 ± 4.0%, compared to the Standard Model prediction of 2%. This is more than 3 sigma discrepancy! Not only we get a novel 3 sigma anomaly to cherish, but we also get a hint of anomalous top polarization.

To wrap up , D0 has brought some exciting news and some worrying news too (see the paper for more worries concerning modeling additional QCD radiation that I didn't mention here). The new results will somewhat shake the hierarchy of new physics models that address Tevatron's anomalies, but we have to wait for the next load of theory papers for quantitative details. On the experimental front, next year Tevatron will update all these measurements with twice as much data. About the same time, the LHC will be seriously joining in the game too. Although the LHC cannot measure the top asymmetry directly, due to the symmetric p-p initial state at the LHC, they can access the same physics by constructing more fancy observables. For example, a recent CMS note investigates whether top quarks move closer to the beam axis than anti-top quarks more often than the other way around. Such an effect would be a consequence of a positive forward-backward asymmetry of the t-tbar pair production in quark-antiquark collisions. No effect has been observed in CMS or ATLAS who studied a similar observable. However this doesn't mean much yet: most models addressing Tevatron's top anomaly predict the CMS observable to be the edge of their current sensitivity. It is more worrying that LHC observes no anomalous effects in other top quark observables, like the production cross section or the invariant mass distribution. Yeah, the obstinate lack of new physics at the LHC is utterly worrying. Guys, you better find something fast, otherwise there'll be nothing but darkness.

The D0 paper is available on arXiv. See also Tommaso's comments on the CMS note.

Higgs won't come out of the closet

2011-07-22T16:27:00.030+01:00

Today we had a true fireworks display in Grenoble: at the EPS conference the LHC experiments presented their Higgs search results based on 1 inverse femtobarn of analyzed data. The cream of the cream is these 2 plots:

Here is just a few fleeting remarks.

There was a significant chance to catch him yet the bastard escaped once again. However it's becoming increasingly clear that this year we will learn whether the Standard Model Higgs exists or not.
CMS excludes the Standard Model Higgs in the 149-206 GeV and 300-440 GeV windows (plus a few peeping holes here and there), while ATLAS excludes the 155-190 GeV and 295-450 GeV windows. The low mass exclusion is dominated by the search of the H→WW→2l2ν final state, while the high mass one is dominated by H→ZZ after combining different Z decay channels.
The exclusion range in the low mass region is smaller than expected. Indeed, there are hints of Higgs-like events in the mass range 130-140 GeV. This is nicely visualized in a plot from the ATLAS talk. The excess in the combined plot is driven by a broad excess WW → 2l+MET events. In certain mass regions the excess is amplified by γγ and ZZ→4lepton excesses, and reaches almost 3 sigma significance. CMS also has a 3-sigmish excess in that same region. This could be a fluke, a mismodeled background, or a first glimpse of the real thing. If the latter is true, we may learn it very soon!
We're looking forward to the ATLAS/CMS combination which should be ready for the next big conference: Lepton-Photon in Mumbai. Most of the high-mass region, up to almost 500 GeV, should be excluded by the combination, and it's not impossible that the low-mass Higgs signal will pop above the 3 sigma surface...
Both ATLAS and CMS presented their searches in the ZZ→4l channel. Yesterday CDF tried to launch its own firework - a statistically large excess of 4 events with two Z bosons decaying to 2 leptons each near the ZZ invariant mass of around 330 GeV. However that firework fizzled out, as none of the LHC experiments sees any ZZ → 4l excess in that mass region. Given that, there is no way the CDF result can be due to a Higgs or any other new particle; it's either a bad fluke or mismodeled background.
We can officially announce that Tevatron is out of the Higgs business. Both ATLAS and CMS on its own have much more powerful exclusion limits than the combined Tevatron exclusion from last summer. LHC should collect 3-5 times more luminosity by the end of the year, which will allow them to beat Tevatron's sensitivity also in the mass region near 115 GeV. Higgs hunting has moved to Geneva, for good...

On a different front, LHCb and CMS presented important limits on Bs → μμ branching fraction. Recall that CDF recently saw a 2-sigmish excess corresponding to Bs → μμ branching fraction of (1.8 ± 1) × 10−8, which, in spite of low statistical significance, prompted some excitement among theorists. However, that central value is now excluded by CMS at almost 95% and by LHCb at more than 95% confidence level. So CDF result seems to be just another fluke... bad luck.

-----
On viXra log Phil is doing a great job of keeping us updated in real time on what is going on at EPS; see this post for a royal collection of Higgs plots. Matt Strassler is blogging live from Grenoble (Et tu, Brute?). See also Tommaso's comments on CMS searches. Tomorrow more excitement guaranteed :-)

Another intriguing result about B-mesons

2011-07-13T13:37:00.010+01:00

Today's update on the measurement of the Bs → μμ branching fraction from CDF makes your heart beat a little faster. The Tevatron collider produces huge numbers of neutral Bd and Bs mesons and they're being looking at from every angle in desperate attempts to spot any departures from the Standard Model predictions. One interesting process to look for is when the 2 quarks making a B-meson annihilate, inducing a decay of that meson to a μ+ μ- pair. This process is mediated by flavor changing neutral currents and therefore within the Standard Model it occurs only via loop processes (see the diagrams), as opposed to much more frequent tree-level charged current decays of the b-quarks. As a consequence, the Bx → μμ decays are suppressed by small loop and CKM factors and the branching fraction ends up being tiny, 3×10^-9 for Bs mesons and 10^-10 for Bd mesons, which is below the current sensitivity. At the same time, these decays has been searched for vigorously because it's fairly easy for new physics to mess them up. For example, additional Higgses in 2-Higgs-doublet models, Z-prime gauge bosons, or SUSY particles in R-parity violating models could mediate these decays pump up the branching fraction.

CDF just posted the latest update on that search based on 7fb-1 of data. They pick up pairs of opposite sign muons originating from the same displaced vertex and measure the dimuon invariant mass. If that mass falls into the window of the Bs or Bd meson mass then we have a Bx → μμ candidate. On top of that, several other properties of these events are cooked into a magic potion (called the neural network discriminant by those in the know) to better distinguish the signal from background. See the plot of the number of events in various bins of the NN discriminant as a function of the dimuon invariant mass. A tantalizing excess can be seen in the upper right window of the plot, with 4 observed vs. 0.9 expected dimuon events having a large likelihood of coming from Bs decays. You should not look in the 2nd left window in that row showing a large excess (16 observed, 8 expected) in the bin where they don't expect any signal ;-) Based on the 3 highest bins, CDF estimates the branching fraction of Bs → μμ is (1.8 ± 1) × 10−8, which is about 2 sigma above the expected Standard Model value. The middle row corresponds to events where one of the muons is detected in the forward region, in which case less signal is expected and no excess is seen. The lower row tells you there is no excess of Bd → μμ events.

So is it interesting or not? First of all, it's merely a 2 sigma excess. Secondly, the data do not trace very well the expected background outside the signal window which casts doubts whether CDF has everything under control. Nevertheless, the new CDF result is very exciting in the context of the D0 observation of the anomalous dimuon charge asymmetry. That anomaly is related to a different decay process where two B-mesons decay to *one* muon each. It is however plausible that both anomalies have a common origin, see for example this paper for quantitative estimates of the Bs → μμ branching fraction in concrete models addressing the D0 anomaly.

The best thing is that we'll learn more very soon. The LHCb experiment is well equipped to make the same measurement. At the moment they have over 400 pb-1 of data on tape. Their own estimates suggest they should be able to see a 3 sigma excess if the Bs → μμ branching fraction is equal to the CDF central value. Moreover, ATLAS and CMS may also try to stick a foot in the door. Hold your breath for just a bit longer; in case anyone sees something the rumor will soon be out on blogs ;-)

See also Tommaso's post. The Wine&Cheese seminar will take place this Friday 9pm Europe time.

D0: 4 sigma like-sign dimuon anomaly!

2011-07-01T02:59:00.006+01:00

About a year ago the D0 collaboration announced a surprising result. They compared the number of events with two positive muons and those with two negative muons. Once the contribution from kaons and pions decaying to muons within the detector is subtracted and some instrumental effects are taken into account, the number of positive and negative muon pairs is expected to be the same. Instead, D0 saw a 1% excess of events with 2 negative muons which represented a 3.2 sigma deviation from the Standard Model prediction. Yesterday D0 presented an update of that measurement based on 9fb-1, that is almost the full data set they have on tape. They obtain the asymmetry of −0.787% with an error of about 0.2%. The anomaly has grown to 3.9 sigma!.

The observed dimuon charge asymmetry is most likely due to asymmetric decays of B-mesons. Bottom quarks inside these mesons can decay as b → c μ- ν, and analogously an anti-bottom quark can decay to a positive muon. Most of the time the Tevatron produces pairs of bottom and anti-bottom quarks, each of them dressing into its own B-meson. However, neutral B-mesons can oscillate into its own antiparticles. If this happens, both original b-quarks may end up decaying to same-sign muons. Furthermore, if the oscillation probability violates CP, that is oscillating Bbar → B is more likely than the other way round, then the excess of negative muon pairs may show up. In fact, such an effect occurs within the Standard Model, but the predicted asymmetry is tiny, of order 0.01%. On the other hand, the asymmetry of the size observed by D0 requires new sources of CP violation beyond the Standard Model. Like what? Like Z', W' charged Higgs, KK gluons, or whatever; we would need more clues to guess the right answer.

An important new element in the latest D0 analysis is the study how the asymmetry depends on muon's impact parameter with respect to the primary vertex of the collision. Muons from B-meson decays often have large impact parameters because decay happens picoseconds after production. On the other hand, muons from kaon decays have typically small impact parameters because the mother kaon usually comes straight from the collision point. Thus, selecting events with large impact parameters enriches the sample with dimuons from B-mesons decays. D0 concludes that the dependence of the asymmetry on the muon impact parameter is consistent with the hypothesis that it indeed originates from B-meson decays, and not from some mundane background. Moreveor, the cut on the impact parameter also affects the relative fractions of Bd and Bs meson decays in the dimuon sample (these fractions are about 50-50 without the IP cut, but due to different oscillation parameters more Bd mesons spit muons with large IP). Thus one can put better constraints on separate contributions of Bs and Bd mesons to the asymmetry. The result is this plot:
The axes are the semileptonic decay asymmetries of the Bd and Bs mesons. The pink band is the fit to the observed dimuon asymmetry without the IP cut, while the ellipse takes into account the input from the IP measurements. Unfortunately, we still cannot tell whether the asymmetry is due to Bs mesons, or Bd mesons, or both, which is of primary importance for theoretical interpretations of the anomaly.

So have we discovered new physics yet? Alas, recent history teaches us not to celebrate before the signal is confirmed by an independent experimental group. The CDF collaboration had an anomaly even larger than 4 sigma which did not stop D0 from ruthlessly shooting it down. The rules of the wild west suggest that CDF may attempt the same with the D0 pet anomaly, after which they all meet at the O.K. Corral. But maybe this time it'll be different? Maybe this time it's for real? We may learn more later this year, either from CDF or from the LHC. Actually, the LHCb experiment promised to deliver a complementary evaluation of the B-meson decay asymmetries by measuring the B →D μ ν decay rates. Because of systematic effects they find it easier to determine the difference of the Bd and Bs meson semileptonic decay asymmetries (while the D0 dimuon asymmetry depends roughly on the sum thereof). With 1fb-1 of data their expected sensitivity corresponds to the thin gray band in the plot on the right. One more reason to bite our nails while waiting for the next LHC results!

Meanwhile at the LHC

2011-06-20T00:35:00.009+01:00

The excitement about the CDF bump is subsiding so we can relax and look back at the LHC. A lot is going on there, although the best of the action will occur later this summer.

As everyone knows, the luminosity collected by ATLAS and CMS just passed the 1 fb-1 benchmark which had originally been the goal for the entire 2011. The LHC management is running a policy of making careful projections which can later be spectacularly surpassed. We all agree it's better than the previous policy of bold projections and spectacular catastrophes.
Unofficial predictions for the luminosity at the end of 2011 are Gaussian distributed with a peak around 5 fb-1. It means that, this year we'll probably learn whether the Standard Model Higgs exists or not! That's massive.
Several results using a good chunk of this year's data, around 200 pb-1, have already emerged from ATLAS on the occasion of the PLHC conference in Perugia 2 weeks ago. The most interesting result is the t-tbar invariant mass spectrum. The most disappointing, too. If Tevatron's excess in the top quark forward-backward asymmetry is due to a 1-2 TeV heavy Kaluza-Klein gluon the t-tbar spectrum should display a peak or an excess at the tail. Alas, nothing there, as you can see.
About the update on searches for Z' decaying to leptons, see Tommaso's blog. There's also a new search for W' in the muon + missing energy channel. In those cases also nothing, only vague rumors.
Meanwhile SUSY searches are continuing at full steam. New analyses in the jets+met and jets+lepton+met channels are out. From the plot you can read that, for equal squark and gluino masses, the limit on the masses is above the magic threshold of 1 TeV. If the squarks are decoupled the limit on the gluino mass is slightly less stringent, about 750 GeV, and similalry the other way around. Children take about 6 years to realize Santa Claus does not exist, the LHC may be quicker than that.
Theorists, on the other hand, are trying to understand the deeper meaning of the LHC limits on SUSY. The conclusion is that SUSY must be just behind the corner, just a little bit more, one last effort, and we'll see it. One should note that preference of the global fits for light superparticle masses is driven by one measurement: the long standing 3 sigma excess in the muon anomalous magnetic moment. Interestingly, a recent paper reevaluates the theoretical contributions to the muon g-2 and concludes there is no excess whatsoever. I am not in a position to judge whether the paper is correct, drop your comment if you are.
Meanwhile, ATLAS and CMS keep posting papers on arXiv which use only the meager last year's harvest of 35pb-1. At this point it feels like offering ZX Spectrum in an Apple store.

The next big dump of LHC results is bound to happen for the EPS conference in Grenoble end of July. Expect numerous new physics analyses using 1.x inverse femtobarn of data. It'll rock!

D0: no bump

2011-06-10T15:52:00.011+01:00

This result has been known to all blog readers since yesterday, now it's officially out. The D0 collaboration just released a new analysis of the dijet invariant mass spectrum in W+2j events using 4.3 fb-1 of data. Recall that CDF, looking at the same final state, saw an unexpected bump in the dijet spectrum near 150 GeV which might be a sign of new physics beyond the Standard Model. D0 closely follows the CDF analysis. Their conclusion: no bump.
They place the 95% CL limit on the cross section of a hypothetical 145 GeV dijet resonance at the level of 2 picobarns. On the other hand, CDF estimates that fitting their signal requires a larger cross section, of order 4 picobarns, which D0 excludes at 4 sigma. It's not straightforward to draw firm conclusions from these numbers. Although both experiments use almost identical kinematic cuts, they have different detector response, quality cuts, etc., which leads to different results. For example, D0 has about 30% more events than CDF near 150 GeV (after scaling with luminosity). It would be illuminating to run realistic models explaining the CDF bump through both detector simulations in order to compare the efficiencies. That would allow us to quantify the discrepancy between CDF in D0 (it may well be much smaller than 4 sigma). In any case, not seeing any excess in the D0 data puts a lot of strain on any new physics explanation of the CDF excess.

For the moment I don't see any catch in the D0 analysis. Previously, D0 employed differential reweighting to model the W+jets background which might have scaled away any excess; however, the present analysis avoids this step. Also, the D0 data match the Standard Model predictions much better than the CDF ones, also away from the 150 GeV region, which suggests they have backgrounds under better control. Looking closer at the plots, it seems the main difference between the 2 experiments is their estimate of the QCD multijet contribution (kudos to Jay for that observation), but at this point it is not clear if this can explain the CDF bump.

In conclusion, the 2 Tevatron experiments got into an epic standoff. One holds 4 aces in his hand. The other says it's a cheat. We need a shootout to decide who's right :-)

The story is of course all over blogs, see e.g. Tommaso, Michael, Georgios, Peter, Lubos, Sean for more coverage.

More details about the CDF bump

2011-06-06T11:06:00.010+01:00

These days the next round of LHC results is beginning to emerge but for at least another week all eyes will be on the Tevatron. Recall that the CDF collaboration is observing an unexplained bumpy feature in the invariant mass spectrum of jet pairs in events with a W boson. Last Thursday CDF released an online note describing the latest update based on 7.3 fb-1 of data. The significance of the excess has increased to 4.1 sigma. The note describes a number of additional checks that the authors of the analysis have made to exclude background mismodeling as the origin of the bump. In particular, it seems that neither the standard model top quark, nor a difference of jet energy scales between quark and gluon jets can be responsible for the excess. Moreover, the excess persists (albeit a bit smaller) when a different Monte Carlo program is used to simulate the background. All in all, currently none of the known sources can explain the peak in a way consistent with all data. It must be a more subtle detector effect, or new physics.

Furthermore, the note presents a number of kinematic distributions of the events in the
window 115 < M_JJ < 175 GeV where the excess is the largest (thanks guys!). One plot makes you jump in your chair: It shows the invariant mass of the sum of the 4-vectors of the 2 jets, the lepton, and the neutrino, the latter reconstructed from the missing energy. If they all originate from one mother resonance, as suggested by a class of models, the invariant mass should reproduce that resonance mass. Indeed, the plot above shows a clear excess just below 300 GeV. This hints at another heavy particle being produced at the Tevatron, which then decays to a W boson and a 150 GeV particle who is directly responsible for the dijet bump. However, the plot on the right, which shows the same distribution but without subtracting the background, tells you that the standard model also peaks around 300 GeV (as a results of the imposed cuts) which makes the peak less trustworthy. If not for that, we would already be dancing on the streets and indulging in wild orgies.

It's also worth looking at another plot showing the transverse mass of the lepton + neutrino system. If the two come from a decay of a W boson, as tacitly assumed in the analysis, that distribution should have an endpoint at m_W = 80 GeV. Since most of the excess is below 80 GeV we can conclude that most of the times the 150 GeV resonance is accompanied by a genuine W boson. This excludes a more exotic class of models I mentioned where the lepton and the neutrino originate from the same particle that is responsible for the bump.

What's next? We are of course dying to hear the story from D0 and from the LHC experiments. The update from D0 is imminent. We expect it to be announced on June 10 at the Wine&Cheese seminar in Fermilab (as a general fact, wine facilitates communication between experiment and theory). As for the LHC, a blog post on Quantum Diaries points to an ATLAS note based on 33 pb-1 of data which roughly repeats the CDF analysis and finds no excess. However this means nothing: ATLAS simply had no right to see the hypothetical CDF bump in their 2010 data. First of all, the larger production cross section at the LHC (5 to 40 times, depending on the production mode) combined with the better efficiency does not make up for the 200 times smaller luminosity. Moreover, the W+jets background at the LHC is about 40 times larger. For these reasons you need a larger data set to make any conclusive statement. Suppose the CDF excess indeed originates from a 300 GeV mother resonance. If that resonance is produced by gluon-gluon collisions then the LHC should be able to see the excess already in 200 pb-1, which is the amount of data used in the most recent analyses. I'm sure hundreds of people are looking into this as we speak and as soon as the peak appears it will be pasted into Peter Woit's blog ;-) If, on the other hand, that resonance is produced in quark-antiquark collisions then we need to wait for 1 inverse femtobarn to see a significant excess at the LHC. In any case, the floor should be swept by the end of this summer, one way or another.

CDF: Wjj bump almost 5 sigma!!!

2011-05-30T21:34:00.014+01:00

Today at the conference Rencontres de Blois the CDF collaboration presented an update on the invariant mass of 2 jets produced in association with a W boson. Recall that 2 months ago CDF posted a paper based of 4.3 fb-1 of data claiming that this observable displays an unexpected bump near 150 GeV with a significance of 3.2 sigma. The bump could have been a fluke, an accounted for systematic effect or surprising new physics. Now the first option is no longer on the table: the same bump is also present in the more recent data with a large statistical significance. With 7.3fb-1 of the Tevatron data, after subtracting the known Standard Model backgrounds other than the WW and WZ production, the distribution of the jet pair invariant mass looks like this:The peak has become more pronounced! CDF quotes the significance of 4.1 sigma (the number 4.8 sigma I quoted earlier takes into account only statistical uncertainties; after including systematic uncertainties the significance drops to 4.1 sigma). In a collider experiment, such a huge departure from a Standard Model prediction is happening for the first time in the human history :-) I don't have to stress how exciting it is. However we're not celebrating the demise of the Standard Model yet, not before an independent confirmation DZero or from the LHC. In any case, this summer is going to be hot.

For possible theoretical explanations of the bump, see my previous badly timed post. In the Blois slides CDF adds one important new piece of information. They say the bump cannot be due to the Standard Model top quark background, contrary to what was suggested in a couple of theory paper. Basically, there is no sign of enhanced b-jet content in the excess events, and in any case the top quark endpoint would show up below 150 GeV due to different jet energy scale corrections for b-jets.

Update: CDF has released more plots and the note describing the update.

Theorists vs. the CDF bump

2011-05-30T14:53:00.013+01:00

Almost 2 months ago the CDF collaboration published their analysis of the events with exactly 2 jets, 1 lepton, and missing energy. These are vastly dominated by boring Standard Model processes where the W boson is produced together with jets and subsequently decays to an electron or a muon and a neutrino. A surprising feature showed up in the distribution of the invariant mass of the jet pairs. After subtracting the Standard Model background, CDF observed a bump near 150 GeV with a significance of 3.2 sigma. Obviously, theorists rushed to interpret the bump in term of physics beyond the standard model. The CDF result hints to a new particle with a mass of around 150 GeV, a significant coupling to the light quarks and a tiny coupling to leptons; the remaining details are left up to our imagination . Here is a selection of the educated guesses that appeared in about 50 papers to date.

The first thing that comes to mind is Z' - a new neutral gauge boson coupled to the left-handed quarks. This is a valid possibility provided Z' is leptophobic, that is to say, its coupling to electrons is less than about 0.05 to avoid constraints from the LEP experiment. There is some tension with the constraints from the UA2 experiment that was operating some 30 years before christ and made a search for a narrow Z' in the dijet channel. The UA2 limits on the Z'-quark coupling translate to a constraint on the W+Z' cross section at the Tevatron that allows one to explain only about 60 percent of the events observed by CDF. However, given the large uncertainties involved in the CDF measurement and in interpreting the UA2 results, the Z' option remains open. One should also note that nothing in the data tells us the new particle is a vector boson, it could just as well be a scalar.

To ease the UA2 constraints one can turn to another class of model. Quite generally, the Tevatron may produce a ≥ 250 GeV mother resonance who decays to a W boson and a 150 GeV daughter resonance. The latter subsequently decays to 2 jets who are observed by CDF. Several proposals for the mother and daughter exist: a technirho meson decaying to a technipion and a W in a version of technicolor, a sbottom decaying to a stop and a W in R-parity violating supersymmetry, a charged Higgs decaying to a neutral Higgs and a W in two-Higgs doublet models, a weak doublet color octet in the Manohar-Wise model, etc. The striking prediction of this class of models is that not only the invariant mass of the jets but also of the entire final state should display a resonance. CDF looked at the invariant mass of the 2 jets + lepton + missing energy vector and found it consistent with background only, but it is not clear if this excludes the presence of a mother resonance (the presence of the missing energy introduces larger systematic uncertainties than for the jet pair mass).

One can also imagine a more intricate class of models where the lepton and the missing energy in the CDF excess events come not from a usual W boson but from some other particle decaying to an electron and a neutrino. For example, this paper explains the excess by a production of a pair of supersymmetric winos of which one decays, via R-parity violation, to a charged lepton and a neutrino, and the other decays to 2 jets. This possibility may be excluded by analysis the distribution of the transverse mass of the lepton+missing energy subsystem.

Finally, one should mention those who are trying to spoil the party. From the very beginning many have cast doubts on the CDF analysis as it requires a perfect control over the overwhelming Standard Model backgrounds. One thing is that even the Standard Model W/Z peak in the observed jet mass spectrum, arising due to the well known contribution of the WW and WZ production processes, does not seem to be very well described by the simulations. Furthermore, by eye it seems that shifting the jet energy scale a few percent upwards, which would correspond to shifting the whole data curve to the right, allows one to get rid of the excess (the authors of the analysis reply that raising the jet energy scale makes additional events pass the analysis cuts, so that naive shifting of the curve is not correct; they say a 3 sigma excess persists even when the JES is scaled up by 7 percent). Another attempted explanation is that the apparent excess is in reality the Standard Model top quark. When a top quark decays hadronically, t → W b → jjb, the invariant mass of the 2 light jets of course peaks at the W boson mass of 80 GeV, however the invariant mass of the b-jet and one of the light jets has the distribution peaking near 150 GeV (the endpoint is Sqrt[mTop^2- mW^2] = 155 GeV), suspiciously close to the CDF bump. Thus, the excess may be due to the semileptonic t-tbar or single top production where one or more additional jets are missed at the detector, assuming the Monte Carlo simulations of that background have been (rather grossly) mismodeled.

So this is where we stand today. The situation may or may not be clarified when more data arrive. The updates from CDF and D0 are imminent. Someone will call a bluff? Or someone is holding an ace up his sleeve? Stay tuned for the next episode.

AMS is on

2011-05-23T10:44:00.015+01:00

AMS-02 is up and running, and first events have already been twitted to the Earth. AMS is a full fledged particle detector attached to the ISS whose goal is to measure the cosmic ray spectra. The mission has been plagued by ill fate (delay due to the Columbia crash, scrapping of their superconducting magnet), now the road seems to be clear at last. The final preparations and the launch have been widely reported in the mainstream media, however my impression was that the actual science that AMS may accomplish was not clearly exposed. Here is my understanding of what AMS could teach us.

The official page of AMS lists the following scientific goals

Search for primordial antimatter
Search for dark matter
Search for exotic forms of matter
Study of the cosmic ray composition

The first point situates somewhere between Sam Ting's fixation and crackpottery. AMS will search for anti-helium nuclei arriving from the outer space. Unlike antiprotons, positrons and anti-deuterium, heavier anti-nuclei are not expected to be produced by cosmic ray collisions; anti-helium would have to be produced by astrophysical objects made of anti-matter. The problem is that we know there is no such thing: all the primordial antimatter annihilated with matter around 1 sec after the big bang. This view is not only the consequence of the current cosmological model, but it is also firmly supported by several independent observations, such as the cosmic gamma-ray spectrum, the cosmic microwave background, and the near perfect agreement between the predictions of nucleosynthesis and the composition of visible matter in the universe. Given the current body of evidence, AMS has a better chance for a 3rd degree encounter than for finding primordial anti-matter.

The situation with dark matter is more subtle. The PAMELA and FERMI satellites launched in the previous decade have been providing us with precise measurements of the high energy cosmic ray spectra. One thing we definitely have learnt is that it is painstaking to search for dark matter this way. Several excesses over theoretical predictions have been reported so far: PAMELA's positrons, Fermi's electrons, Fermi's photons from the galactic centre. They all have a plausible interpretation in terms of models of dark matter and an equally plausible interpretation in terms of boring astrophysical phenomena. AMS may provide more input regarding the high energy spectra. As can seen in the plots of the projected sensitivity, after 10 years of data taking they expect to extend the measurement of the positron and antiproton spectra up to almost TeV (compared to the current reach of PAMELA of about 200 GeV). It's hard to say if these projections are realistic, since it is not clear how much the resolution at high energies is degraded due to the replacement of the superconducting magnet by a weaker permanent one. Assuming they are realistic, particle physicists will be able to refine their models of dark matter, and astrophysicists to refine their models of pulsars. In any case, the chances for a smoking gun signal of dark matter appear slim at this point.

Nevertheless, there is one area where AMS is clearly superior to all previous experiments. The instrumentation of AMS includes a calorimeter, trackers, a Cherenkov detector and a time-of-flight detector to measure the energy, charge and mass of incoming particles. All this gives them very good particle identification, in particular they can easily separate heavier nuclei from much more numerous protons and helium nuclei. Flux ratios of various heavy nuclei, for example the boron-to-carbon ratio, are an important input for the models of cosmic ray production and propagation. Furthermore, if there exists exotic matter with distinct charge-to-mass ratio, for example the hypothetical strangelets with small Z/A, AMS is well equipped to identify it.

In summary, high energy astrophysics is a crowded field, and AMS is unlikely to turn it upside down. Their best shot for a spectacular discovery is exotic forms of matter with distinct Z/A ratio, provided they exist. Furthermore, if AMS and the ISS last long enough, and if the performance of the detector is as good as they promise, they should be able to extend PAMELA and FERMI measurements of the antiproton and positron spectra to higher energies, which may or may not clarify the origin of the positron and electron excess in PAMELA and Fermi. In the worst case AMS will sort out the spectra of heavier cosmic ray nuclei, providing valuable input for cosmic ray propagation model. Critics may complain that 2 billion dollars for tuning GALPROP is a lot. Optimists may stress that so far it's the only hope for returns from the 200 billion dollars sunk into the ISS.

Figures are taken from the talk of Andrei Kounine at TeVPA'10.

Fermi confirms PAMELA

2011-05-17T00:29:00.008+01:00

The annual sabbat of the Fermi collaboration took place last week in Rome. One of the new results presented there was the measurement of the positron fraction in the cosmic rays. This is a very hot observable given PAMELA's claim that the positron fraction at high energies is larger than the one predicted by models of cosmic ray propagation. The PAMELA excess can be interpreted as a signature of dark matter, although boring astrophysical explanations are also possible. But a skeptic could doubt the PAMELA result. Measuring the positron fraction requires discriminating between positrons and much numerous protons; one needs the proton rejection power at the level of 1 in 100 000. PAMELA claims to have that rejection power, but the possibility of an unaccounted for systematic effect did exist.

Now the PAMELA result has been confirmed by a cute measurement performed by the Fermi satellite. Fermi, unlike PAMELA, does not have a magnet to distinguish positively charged particles from negatively charged ones. That's why, until now, they were presenting only the combined flux of electrons and positrons. Nevertheless, they are able to some extent separate electrons and positrons by borrowing the magnet from the Earth. The configuration of the Earth magnetic field lines happens to be such that for certain energies and certain arrival directions only electrons or only positrons are expected, see the picture. Using this effect, Fermi was able to produce the following measurement of the positron fraction:
The black data points are from PAMELA, and the grey band is the Fermi measurement with their systematical uncertainties. The two are nicely consistent. So, we still don't know whether the positron excess is due to dark matter or pulsars or old newspapers, but at least we know for sure it is real.

Another result presented in Rome deserves some advertisement. One of the main goals of Fermi is to search for gamma rays produced by annihilation of dark matter. A good strategy is to look in the direction of one of the dwarf galaxies. These are small satellites of our galaxy that are vastly dominated by dark matter, therefore the annihilation signal can be significant while the astrophysical backgrounds are less pesky. So far, no anomalous gamma ray flux from dwarf galaxies has been detected. From that, Fermi is able to put quite stringent limits on the annihilation cross section for various hypotheses about the final state into which dark matter annihilates:
The point is that for certain hypotheses, like for light dark matter annihilating into tau leptons or b-quarks, they are already excluding the cross sections expected if the dark matter is a thermal relic. So they're really closing in on the parameter space where a signal may be lurking if the WIMP paradigm is true. No luck so far, but they'll try again next year :-)

CoGeNT observes annual modulation!

2011-05-05T17:12:00.013+01:00

What a year... Previously I had to think hard to make up a blogging subject that would not be too boring. But these days there's hardly a week without a new discovery, a new rumor of a discovery, or a refutal of the previous week's rumor. This year the particle community was already electrified by the CDF forward-backward asymmetry, the CDF W+2j bump, the would-be Higgs decaying to photons in ATLAS, and now there is CoGeNT... The rumor that CoGeNT observes the annual modulation of the signal has been circulating for a while, but only recently it was officially announced, first at the APS April Meeting in Disneyland last Monday, and today at the symposium in STSI Baltimore.

CoGeNT is a dark matter experiment located in the Soudan mine in Minesotta. In spite of a relatively small size and limited background rejection its germanium detector has certain advantages, e.g. a low threshold (0.4 keVee, corresponding to true recoil energy of about 2 keV) and a very good energy resolution. This makes it particularly sensitive to light GeV-scale dark matter whose scattering cannot produce nuclear recoils far above keV. CoGeNT was running continuously since December 2009 until March 2011 when the power was cut off due to a fire in the Soudan mine. The new results based on 145 kg.day of data continue to show an excess of events at low recoil energies which can be interpreted as the scattering of light dark matter particles in the detector. The preferred parameter region has shrunk and now points to 7-8 GeV particle with the cross section on nucleons around 10-40 cm2. More importantly they were able to measure the annual modulation of the signal. Because the velocity of the Earth with respect to the dark matter sea changes anually due to the orbital motion around the Sun, the event rate of dark matter scattering is expected to oscillate with a peak in June and a minimum in December. And here is what CoGeNT observes.
The solid line is the expectation from dark matter, and the dashed line is the best modulation fit to the signal. The phases of the two are within 1 sigma. CoGeNT estimates that the modulation hypothesis is preferred at 2.8 sigma. The modulation is most pronounced in the 0.5-2 keV region while it is absent for surface events.

Well, I don't know what to think about it. The parameter region consistent with the CoGeNT signal is naively excluded by CDMS, Xenon10 and Xenon100. One would have to assume that these 3 experiments are terribly wrong about their energy scale in order to reconcile their limits with the CoGENT signal. Maybe CoGeNT is just wrong. On the other hand, the observed modulation is very intriguing, especially in combination with the long-standing DAMA modulation signal and the oxygen band excess in CRESST. On the third hand, maybe nobody is wrong, but dark matter is simply different than what we've expected it to be. Prepare for a new wave of dark papers on arXiv.

The video of CoGeNT's presentation is here. See also this post on Cosmic Variance.

ATLAS shrugged

2011-04-28T00:57:00.011+01:00

A new rumor propagating via twitter, social networks and other attributes of modern civilization says that the old rumor was just a rumor. The ATLAS collaboration was working hard this last week to investigate the possible signal of Higgs decaying to photons. Observations of such a signal was claimed in an internal memo written by 4 of its members. However, after analyzing about 100 pb-1 of currently available data ATLAS concluded that the suspicious excess in the γγ invariant mass distribution is going away. C'est la vie, as the local folks would say. A parallel rumor says that CMS sees nothing unusual in the γγ channel. That would pretty much end the career of the first LHC rumor. Maybe it's for the better. Otherwise the Higgs boson would have to be renamed to the Wu boson which could lead to some confusion.

Anyway, same time tomorrow, we must have a winner one day...

Update: ATLAS officially quashes the rumor. With 131 pb-1 of data there is no peak in the γγ invariant mass spectrum near 115 GeV.

Higgs in ATLAS, maybe

2011-04-22T02:05:00.020+01:00

Peter's blog breaks a story that will be all over the news tomorrow. The Easter Bunny dropped in his comment section the abstract of a sensational ATLAS internal note, which says

...we studied the γγ invariant mass distribution over the range of 80 to 150 GeV/c2. With 37.5 pb−1 data from 2010 and 26.0 pb−1 from 2011, we observe a γγ resonance around 115 GeV with a significance of 4σ. The event rate for this resonance is about thirty times larger than the expectation from Higgs to γγ in the standard model...

So, the note claims no less than a firm evidence for a 115 GeV Higgs boson decaying into 2 photons. This decay occurs in the standard model with the amplitude dominated by a W-boson loop. However, this decay is very rare - only about 0.2 percent of Higgses decay this way. It's absolutely impossible that ATLAS is seeing the standard model Higgs - the rate is way too small. However, one can imagine theories beyond the standard model where the production cross section of the Higgs and/or its branching fraction into 2 photons is enhanced.

Well, it's past 4am, I really need to sleep. So just a few hasty remarks

The H to γγ rate 30 times larger than the standard model one appears to be in tension with the existing Tevatron bounds. Both CDF and D0 in this mass range place a limit of order 20 times the standard model cross section. Actually, D0's limits shown at Moriond display an intriguing 2-sigma excess around 115 GeV...
ATLAS also showed their first H to γγ results at Moriond. At that time, their limits were about 40 times the standard model rate. Actually, a bumpy feature near 115 GeV can already be discerned, especially if one assumes that the background rate is overestimated...
Enhancement of the H to γγ rate by 20-30 times sounds humongous. This is certainly not a feature in any mainstream model, and I'm not aware of a single theory paper that predicts it. A recent paper argues that in the MSSM the rate is usually suppressed; in the NMSSM an enhancement is possible, however by a factor of 2 rather than 20. One ancient paper studies this issue in a more general manner. Large enhancement of the branching fraction to 2 photons is possible if the Higgs couples only to up-type quark (h0_u), or if it does not couple to fermions at all, the so-called fermiophobic Higgs (h0_bh). But this is still not enough; one would need in addition new charged particles with a large coupling to the Higgs (4th generation?, composite fermions?) in order to pump up the production rate.
Strictly speaking, ATLAS observes a particle decaying to 2 photons, likely a scalar, and likely produced in gluon fusion. A coupling to photons or gluons is not a defining property of the Higgs boson. To demonstrate the Higgsesness of the new particle one would have to pinpoint its coupling to electroweak gauge bosons, for example by measuring its associated production with W and Z bosons. Until that is done, alternative options are on the table.

Whatever it is, it means busy days ahead, for theorists and experimentalists alike. One thing is certain: it's not a hoax, the note and the analysis really exist. But one should keep in mind that the result has not been internally reviewed yet, thus, at this point, it is not backed by the entire ATLAS collaboration. It may well turn out to be a false alarm... or it could be the discovery of the century... stay tuned.

Update: Tommaso points to newer and more stringent CDF limits than I linked to above. For a 115 GeV Higgs the γγ rate must be less than 15 times the standard model rate, which further disfavors the ATLAS signal. On the other hand, CDF has some excess near 120 GeV...

Xenon100: Nothing

2011-04-14T14:13:00.012+01:00

The most expected experimental result of Spring 2011 is out now. XENON100 just released the results of the dark matter search based on 100 days of data-taking with xenon target in 2010. Here is what they see:
The plot shows all events that pass the quality cuts. The x-axis corresponds to the measured recoil energy determined by counting the number of scintillation photons in the event, the so-called S1. (There is an important companion paper fixing the relation between recoil and S1 at low energies where previous experimental results have been somewhat confusing). Most of the events in the plot are due to photons scattering on electrons from the xenon atoms. The way to distinguish those from the more interesting nuclear recoils (expected when a dark matter particle scatters) is by simultaneously measuring the number of ionization electrons, the so-called S2. Nuclear recoils typically lead to a smaller ratio of S2/S1 (the grey area in the plot). Therefore one makes a cut on S2/S1 (the dashed horizontal line) defining the signal region such that most electron recoils are rejected while the bulk of nuclear recoils is retained. At the end of the day one finds 3 events in the signal window (red points) while the expected background, mostly from spillover of electron recoils, is estimated to be 1.8 ± 0.6. Once again, no signal :-( Instead, we have new limits on the dark matter - nucleon cross section
For a 100 GeV dark matter particle the limit is around 10^-44 cm2, 3 times better than the previous limits from CDMS and Edelweiss. For light dark matter the improvement seems to be even better, more than an order of magnitude, which further disfavors dark matter interpretations of the CoGeNT and DAMA signals.

Actually, the paper mentions in passing that the analysis leading to these limits was not completely blind. After opening the box, there were many events at small recoil energy of which 3 fell into the signal region, which would make 6 signal events in total. However after investigating these 3 additional events the collaboration decided they were static from the electric can opener ;-), and devised additional cuts to get rid of them.

So what do these results tell us about the WIMP dark matter? At which point should we start to worry that we're on the wrong track? Unfortunately, there is no sharp prediction for the dark matter cross section. The most appealing possibility – a weak scale dark matter particle interacting with matter via Z-boson exchange - leads to the cross section of order 10^-39 cm2 which was excluded back in the 80s by the first round of dark matter experiments. There exists another natural possibility for WIMP dark matter: a particle interacting via Higgs boson exchange. This would lead to the cross section in the 10^-42-10^-46 cm2 ballpark (depending on the Higgs mass and on the coupling of dark matter to the Higgs). This generic possibility is now getting disfavored thanks to Xenon100's efforts, unless the Higgs is heavier than we expect. Therefore, even though models predicting the cross section below 10^-44 cm2 certainly do exist, it may be a good moment to start thinking more seriously about alternatives to WIMP. In the worst case dark matter may be very weakly interacting (axions, gravitinos) or very light (keV-MeV scale dark matter), in which case the current approach to direct detection is doomed from the start.

See also Peter's and Tommaso's blogs.

Another 3 sigma from CDF

2011-04-06T21:38:00.017+01:00

Gosh, I was damn busy all day, and by now all major blogs have already run the story. Anyway, better late than never... CDF claims a 3 sigma excess in the dijet invariant mass distribution in the lepton + neutrino + 2 jets events. This analysis was originally devised to search for the diboson WW and WZ final states, where a W boson decays leptonically and the other electroweak boson decays into 2 jets. The latter should show up as a broad peak in the dijet invariant mass spectrum, on top of a much larger background from the generic W+jets production. Indeed, both D0 and CDF could see the peak below 100 GeV, which allowed them to pinpoint the diboson production at the 5 sigma level and measure the diboson production cross section. However, CDF had also a small blip around 150 GeV which was not expected.

All in all, the excess advertised in today's CDF paper is not exactly new, and it has been widely discussed among theorists. Moreover, the analysis published today has been publicly available for some time in the form a PhD thesis. What changed with respect to the earlier CDF diboson search is that the cuts have been slightly revamped to make the bump more pronounced. The excess over the standard model prediction is estimated to be slightly above 3 sigma.

It is well known that sigmas come in varieties: there or more significant 3 sigmas, less significant 3 sigmas, and astrophysical 3 sigmas. To my taste the latest CDF claim belongs to the 2nd category. We are dealing with a small hump on top of a huge background, and a small unaccounted for systematic error could easily show up as a false positive. Furthermore, D0 does not see anything; they actually have a small deficit near 150 GeV (although with much less data and different cuts). Finally, the recent experience with papers submitted simultaneously to arXiv and to popular news outlets is not quite encouraging ;-)

In spite of these caveats the effect is definitely interesting and we cannot exclude it is real. What could it be? It is not a Higgs; anything Higgsish with 150 GeV mass would prefer decaying to a pair of W bosons or b-quarks rather than to 2 light jets. The simplest explanation, proposed in this April Fools' paper, involves a 150 GeV Z' boson. A light Zprime with a significant coupling to leptons is excluded by LEP and the Tevatron, but if the coupling to leptons is small then the limits are surprisingly weak. In particular, 150 GeV Z' with electroweak size couplings to quarks is perfectly allowed, and would have the right cross section to produce a bump observed by CDF. One should note that Z' with the mass of that order could generate a large forward backward asymmetry of the top production, as observed in another CDF analysis. But one should also note that generating the asymmetry requires a large flavor violating coupling u t Z' which in principle is not related to the coupling to the light quarks that is probed by today's CDF paper. In a month or so, when I post an update, there will surely by dozens of new models explaining the bump, and some of them may link to the top asymmetry in a more direct way.

For more comments see Flip, Tommaso, Sean, and Michael, Peter, Lubos, and again Tommaso.

Update on forward-backward asymmetry

2011-04-04T01:47:00.010+01:00

Even though the LHC is flooding us with new results, Tevatron's forward-backward asymmetry of the top-quark pair production remains the most intriguing collider result as of today. The effect hints at an exciting new physics at or below the TeV scale which may soon be uncovered by the LHC, or maybe by further analyses at the Tevatron.

To visualize what kind of departure from the standard model we need to explain the Tevatron data, see the plot borrowed from this paper The plot shows a model independent fit to the Tevatron top results. The axes are the forward and backward production cross sections of top pairs with the high invariant mass, m_ttbar > 450 GeV. One can see that the cross sections has to be modified by some 10-20 percent, and that the best fit point corresponds to the backward cross section being smaller than the standard model one. This is possible if new physics contribution to the top pair production interferes destructively with that of QCD.

To modify the cross section one can introduce one or more particles beyond the standard model that mess up into the top production. The successful models have to satisfy 2 basic requirements:

the new particle should not be too heavy and should couple strongly enough to both the 1st generation quarks and to the top quarks. Otherwise the effect would not show up on top of the QCD top pair production.
the new particle should couple chirally, that is to say, with a different strength to left- and right-handed quarks. Otherwise it would not generate a forward-backward asymmetry.

Models with these properties have always been around, but their number has been going through an inflationary phase during these last 3 months. One can robustly divide them into 3 classes:

s-channel mediators,
t-channel mediators,
new particles decaying to tops

In the text below I dropped some links to a few interesting recent examples.

In the s-channel models the asymmetry is due to a resonance which can be produced in a q-qbar collision and decays to a pair of top quarks. An example of such a resonance is a Kaluza-Klein excitation of the gluon in Randall-Sundrum-type models, or more generally an axigluon in models where the standard model SU(3) color group is extended. This class is somewhat disfavored by experiment. Since the differential top production cross section measured at the Tevatron agrees very well with the standard model, the resonance has to be either very heavy or very wide. Furthermore, since the resonance has to couple strongly to the 1st generation quarks, it is constrained by the negative results from the LHC resonance and contact interaction searches in the dijet final state.

In the t-channel models the asymmetry is generated by an exchange of an off-shell particle that has a cubic vertex with one 1st generation and one top quark. There are many possibilities for the mediator: it could be a gauge boson or a scalar; electrically charged or neutral; a color triplet, sextet, sexist, octet, and so on. Some of these models survive all experimental constraints, although there is always some tension with the measured differential top production cross section. Another problem with this class is philosophical: the new particle has to have a large flavor violating coupling to the 1st and the 3rd generation, whereas similar couplings to the 1st and 2nd or to the 2nd and 3rd generations appear to be severely constrained by experiment. If any of these models corresponds to reality then some unexpected flavor structure is being revealed.

As for the last class mentioned above, I'm aware of only one model of that kind where a new color triplet scalar decays to a top quark plus a light invisible fermion. Sounds much like stop → top + neutralino, although the required couplings do not fit SUSY. As the new particle production cannot of course interfere with the QCD top production, this possibility is somewhat disfavored by the top cross section measurements. On the other hand, such a new particle can be very well disguised, especially when its mass is close to the top mass, and is currently poorly constrained by new physics searches.

So which model is true? If we're lucky the answer may already be in the 2010 LHC data; however very few top analyses has gone public so far. Otherwise we'll have to wait till the summer conferences. If the LHC continues at this pace we may have 1 inverse femtobarn to play with by that time.

April Fools'11: ATLAS or CMS? One of them must leave!

2011-04-01T02:02:00.016+01:00

The astonishing rumor that has been circulating since two weeks turned out to be true. The CERN management has decided that either ATLAS or CMS will be taken out of operation as of September 2011. Which of these two detectors will be scrapped is going to be decided next week during a special session of the CERN Council. In an email sent yesterday evening to CERN staff members CERN Director General Rolf Heuer explains:

"This was a very difficult decision to make. However, we are going through difficult times and radical steps could not be avoided. In the current budgetary situation we simply cannot afford running two experiments with identical physics goals and similar detection capabilities."

Makes sense, at first glance. CERN is seriously indebted and some austerity measures need to be implemented. Nevertheless, laying off over three thousand researchers seems exorbitant at this point. Here at CERN it is not a secret that this decision was prompted by the growing animosity between the ATLAS and CMS collaborations which is seriously disrupting the LHC operation.

The relations between these 2 experiments have always been strained but they further deteriorated last winter with the arrival of the results from the 2010 run. The spark that triggered the conflict was the fact that the constraints on supersymmetry from ATLAS were systematically more stringent than those from CMS. For laymen it may appear irrelevant whether a 800 GeV gluino is excluded at 95% or at 91% confidence level, but for particle physics experiments these numbers are of primary importance. Consequently, CMS accused ATLAS of using dirty statistical tricks to boost the significance of their result. From that point it all went downhill, and almost every day brought new incidents, such as leaking of hacked ATLAS emails, a bug found in the CMS control room, a libel lawsuit against the ATLAS spokesperson, etc. But the most serious incident happened 2 weeks ago during the Moriond'11 conference in a hotel resort in the Italian Alps. What started as a benign discussion about the missing energy resolution turned into a regular fist fight involving a dozen members of ATLAS and CMS; all that happening in front of terrified tourists and local villagers. Although nobody got seriously hurt, that incident was probably the last straw, and the CERN management decided to put an end to the conflict in the most drastic way.

Now, what will happen to the thousands of researchers who often devoted their entire careers to the experiment? Hopefully, a number of them will be allowed to join LHCb or ALICE who will surely welcome additional manpower. Furthermore, CERN Director General promised that many of those laid off this year will be rehired in 2013 to help training the superconducting magnets, which will be the vital step in the preparations for the energy upgrade in 2014.

Update: This post is of course an April Fools' joke. The research at the LHC proceeds in the spirit of friendly competition. That means, there hasn't been any fist fights between ATLAS and CMS, so far ;-)

The World after Moriond Electroweak

2011-03-22T01:52:00.020+01:00

Rencontres de Moriond is a conference series taking place high in the Italian Alps where particle physics experiments like to present their latest analyses. This year the quality of snow was not quite satisfactory, but the quality of physics results somewhat made up for it. Some highlights have already been discussed on this and other blogs, but I think it's worth giving a short recap anyway.

New SUSY searches from ATLAS and CMS.
Searches in several new channels have been presented: jets+MET+b-tags (relevant for sbottom production), trileptons (relevant for gauge mediated models), e-mu resonance (relevant for certain R-parity violating scenarios), and so on. No excess has been seen, and the parameter space of SUSY has been further constrained. Later I may write something more about the significance of these results, but actually the most striking observation here is how both experiments illustrated their analyses. CMS showed a series of cartoons that pretty accurately summarizes the evolution of SUSY from the early 90s till today:
ATLAS, on the other hand, for the illustration picked a screen from Impossible Mission, a computer game from the 80s. Those who grew up on ZX Spectrum remember well that the game was long, frustrating, and actually impossible to complete ;-)
New Higgs combination from the Tevatron.
In the last couple of years we got used to Tevatron shrinking the available range for the Higgs mass. This time the 95 percent exclusion range is actually slightly worse than in summer 2010 due to an upward background fluctuation. This may be a sign that the Tevatron Higgs searches are approaching the end of the line, and the full data set that will be analyzed next year may not bring significant improvements. See Tommaso's blog for more comments.
First exciting Higgs results from the LHC.
The ATLAS and CMS searches for Higgs-like scalars in the tau-tau final state chops off a new portion the MSSM Higgs parameter space. See this post for more details.
LHC measurements of top quark properties.
Top is at the moment the hottest issue in particle theory due to the anomaly in the forward-backward asymmetry of the top quark production measured by Tevatron's CDF. Most new physics explanations predict new phenomena that should affect various top quark properties measured at the LHC. CMS flashed some plots the very important distribution of the differential top pair production cross section as a function of the pair invariant mass.Nothing unusual can be seen there, except for a small glitch near 700 GeV. This measurement should severely constrain some explanations of the CDF anomaly, for example those involving the heavy gluon partner.

Two additional results are worth pointing out. Both are rather a show-off at this point, but they demonstrate the amazing potential of the LHC experiment and promise interesting physics in the coming year.

CMS observation of single top.
It took 8 years of the Tevatron Run-2 to pinpoint the single top production, and even now it cannot be extracted from the background without some neural network hocus pocus. On the other hand, CMS was able to observe that process with just 35 fb-1 of data. As new physics often modifies the coupling of the top quark to W and b, which in turn affects the cross section for the single top production, single top may provide important constraints or discoveries in the near future. See also this post on Symmetry Breaking.
LHCb limits Bs → μμ.
This is supposed to be the flag measurement of the LHCb experiment. The importance of this rare decay process stems from the fact that the branching fraction is very suppressed in the Standard Model whereas it can easily be enhanced in many theories beyond the Standard Model, in particular in the MSSM. The first LHCb limit is already close to that from the Tevatron, so that LHCb should take over already this summer. See Collider Blog for more details on the measurement.

This week is taking place the second part of Moriond'11 oriented more on QCD, so we are guaranteed another load of interesting results. As for me, I'm dying to see more of dijets and top quarks results.

LHC seriously into Higgs searches

2011-03-15T17:12:00.013+01:00

As of today the Higgs search industry is still dominated by the Tevatron; for comments on the latest results based on 8.2 inverse femtobarn see Tommaso's blog. But the LHC is catching up faster than expected. Yesterday I saw the first interesting Higgs limits from the LHC. At Moriond, both ATLAS and CMS presented the results of the MSSM Higgs searches in the ττ final state. This search is of quite some interest because of earlier reports from the Tevatron: both CDF and D0 claimed a 2 sigma excess for MSSM Higgs searches in another channel with 3 b-quarks in the final state.

In the MSSM, the Higgs sector is extended as compared to the Standard Model. Apart from the Higgs boson there are 2 additional electrically neutral scalar particles. The production cross section of Higgs and its partners can be largely enhanced for large values of the parameter tanβ. Once produced, part of the time the Higgses decay into a pair of τ-leptons. Lacking any observable excess in the ττ channel, CMS and ATLAS thus produced the limits in the tanβ-mA plane:

See that they beat the corresponding Tevatron limits. It is now clear that the Tevatron 3-b excess is unlikely to be explained within the MSSM. However, the excess can still be a hint of a more general extended Higgs sector, for example a non-susy 2-Higgs doublet model.

One more interesting result was presented by ATLAS. This time the search was for a light particle in the 10 GeV mass ballpark decaying to a pair of muons. This could be for example a pseudoscalar Higgs in the NMSSM, a dark Higgs in the hidden-valley scenario, etc. The ATLAS limits display an intriguing bump near 7 GeV. It's probably nothing but a harmless fluke, but it's worth keeping an eye on. Especially if you can keep an eye over a shoulder of an experimentalist ;-)

NEUTEL11

2011-03-15T11:53:00.004+01:00

Contrary to what the name suggests, NEUTEL is not a telecom company but a series of conferences focused on neutrino physics. This year's edition is currently taking place in Venice. For me, the most expected talk is the one from the Xenon100 collaboration who may or may not present their new results of dark matter direct detection searches. For this and other highlights, check out the conference blog that Tommaso is running.

Update: It's confirmed that Xenon100 will not present new results at NEUTEL. We have to bite our fingernails for a few more weeks.

CDF: curiouser and curiouser

2011-03-05T01:56:00.014+01:00

The Tevatron may be a dead man walking but it continues to kick ass. The CDF collaboration just posted a new measurement of the forward-backward asymmetry of the top pair production. Recall that earlier this year CDF made a surprising claim about that asymmetry. Restricting to the top pairs with the invariant mass larger than 450 GeV (about 30% of all t-tbar events) the asymmetry is stunning 48±11 percent, which is 3.4 sigma away from the Standard Model prediction of 9 percent. This is completely crazy: 3 times more often top quarks choose to shoot forward rather than backward (with respect to the direction of the proton beam), even though in the first approximation they should not prefer any direction. Even fancy new physics model have a hard time to predict such a huge asymmetry.

The previous CDF measurement was dealing with semileptonic top decays when one top or anti-top quark decays leptonically to an electron/muon + neutrino + b-quark, while the other decays to 3 quarks. The new measurement that I'm reviewing here is focused on dileptonic decays when both the top and the anti-top decay leptonically. This type of event is more rare; only about 5% of the top pairs decay in this manner. Nevertheless, the top stash at the Tevatron is now large enough. In 5.1 inverse femtobarn that went into the new analysis one expects over 200 dileptonic top events. This is enough to study differential distributions and asymmetries. The CDF note gives the result for the inclusive forward-backward asymmetry, that is for the entire dileptonic sample regardless of the invariant mass of the reconstructed top pair. The measured inclusive asymmetry is 14± 5% which, after unfolding the background and instrumental effects, corresponds to the parton level asymmetry of 42 ± 16 percent. The Standard Model predicts meager 5% so the discrepancy is 2.3 sigma. Much as in the semileptonic sample, the asymmetry is larger at higher t-tbar invariant masses (see the pictures), however poor statistics precludes any firm conclusions. One should also compare that result to the inclusive asymmetry of the semileptonic sample. The latter is much smaller, 16±7%, nevertheless the two results are consistent within 2 sigma.

Formally, the new CDF result is merely a 2 sigma deviation from the Standard Model. However, when combined with the previous 3 sigma anomaly, it has a much stronger psychological impact. One could worry that the CDF measurement of the asymmetry suffers from some unaccounted for systematic effects. In fact, the semileptonic sample has a quirky trait that the entire asymmetry comes from the events featuring a muon, while the events containing an electron do not show a significant asymmetry. Until very recently the following explanations of the anomaly seemed equally plausible:

a cat got stranded in the CDF muon chambers,
the QCD contribution to the asymmetry has been underestimated,
the asymmetry is a manifestation of new physics.

The new result makes the cat hypothesis less likely. In the dileptonic sample the asymmetry is actually the largest for the dielectron events. The systematic effects are quite different for the two measurements, yet both consistently show a large positive asymmetry, which is reassuring.

Now, it remains to make sure that higher order QCD corrections are not playing a dirty trick on us. If not, there will be one option left on the table....

More SUSY limits

2011-02-21T17:01:00.015+01:00

Here in France this time of the year is known as winter holidays among schoolkids, or winter conferences among particle physicists, which amounts to the same thing (unless you're a PhD student working 24h/day to meet the deadline). These days new experimental results pop up like mushrooms with the peak expected middle March at the Moriond conference. Last week new results from LHC SUSY searches were presented at the Aspen conference, both by CMS and ATLAS. The latest additions are the jets+MET search from ATLAS, and the photons+MET and dileptons+MET searches from CMS. The new ATLAS search provides the current best limits on the mSUGRA parameter spaceUnfortunately, for the moment ATLAS and CMS present their theoretical interpretations only in this obscure and contrived way. So it might be worthwhile to discuss the physics behind the above plot and its more general consequences.

At the current stage, SUSY searches are in fact searches for squarks and gluinos, the superpartners of the Standard Model quarks and gluons. That's because only superpartners carrying the QCD charge have had a chance to be produced at the LHC in reasonable quantities. With 45pb-1 of luminosity acquired so far, the LHC is sensitive to cross sections of order a picobarn. As can be read off the plot on the right, this roughly translates to a sensitivity to 600 GeV gluinos and squarks, slightly less than what you might naively guess from the mSUGRA plot. However, if squarks and gluinos have comparable masses one can profit from the squark+gluino associated production which has the largest cross section of all the production channels. The proximity of squark and gluino masses occurs in a large portion of the mSUGRA parameter space, that's why the exclusion limits extend up to 800 GeV masses in this case.

Thus the production processes are relatively straightforward, it's the decay where the supersymmetric hell breaks loose. In the popular SUSY scenarios squarks and gluinos decay to the lightest superpartner, usually a neutralino, who is an electrically neutral stable particle that escapes the detector. In the simplest case, the squark decays to 1 quark + 1 neutralino, while gluino decays to 2 quarks + 1 neutralino (via an off-shell squark). Thus the experimental signature of both squarks and gluinos is a number of energetic jets accompanied by missing energy carried off by the neutralino. This is precisely what is targeted by the jet+MET search in ATLAS and CMS, and it is the most robust signature of supersymmetry. However things can get infinitely more complicated. For example, if charginos (superpartners of W bosons and charged Higgs fields) are lighter than squarks, as is always the case in mSUGRA, a squark may choose to first decay to a chargino who then decays down the lightest neutralino. These cascade decays may spit electrons or muons on the way. Thus, ATLAS and CMS also search for jets and missing energy associated with one, two, or three leptons. This is slightly less robust, as the presence of light charginos is not guaranteed, but at the same time the leptons in the final state help to reduce the Standard Model background. Comparing the recent ATLAS 0-lepton and 1-lepton searches one finds that the former gives slightly better limits on mSUGRA, but that might be different in other SUSY scenarios.

So the cord tightens. With several final states already covered, and more to appear soon, it's getting harder to avoid the stringent LHC limits in most popular SUSY scenario. Nevertheless, the possibility of sub-TeV superpartners has not been completely excluded yet. Firstly, uncolored superpartners are not constrained by the LHC. Furthermore, gluinos and/or squarks with masses 500 GeV or less are still allowed as long as the mass splitting with the lightest neutralino is small enough, such that the supersymmetric events fail the missing energy cuts. Stops, that is the scalar superpartners of the top quark, are even less constrained due to the smaller production cross section and the pesky t-tbar background. As a last resort one can turn to R-parity violating scenarios which are not constrained by the current LHC searches.

To conclude this post and fulfill my weekly quota of malice and scoff, here is a picture shown at the end of the ATLAS talk in Aspen:
Hmmm. Sorry to disappoint you guys but it's not the "stop", it's the "wrong way"; a symbolic mistake in this context. Let's hope future discovery claims from ATLAS will be more carefully scrutinized ;-)

See also Peter's comments.

What LHC tells about SUSY

2011-02-16T02:29:00.016+01:00

The first SUSY searches using the full 2010 LHC dataset are out now, one from CMS and one from ATLAS. The nagging question is what are their implications for low-energy supersymmetry. Some answers can be found in this talk of Alessandro Strumia who provides a cute visualization of the impact of the latest LHC results:
Let me explain what's on this plot. It assumes the so-called mSUGRA scenario which is cherished by experimentalist because it parametrizes the multitude of the MSSM parameters in terms of just 5 variables, thus creating an illusion of order in the Universe. 2 of these variables, the A-term and tanβ, are fixed above to a specific value, the same as the one assumed by CMS and ATLAS in their theoretical interpretations. This leaves 3 variables: the universal scalar mass m_0, the universal gaugino mass M_1/2, and the μ-term (the first two are defined at the GUT scale and related to the physical masses of the SUSY particles by complicated differential equations; this is one of the curious idiosyncrasies of SUSY phenomenology). Quite generally, in the MSSM one can compute the weak scale, that is the Higgs vacuum expectation value, in terms of the parameters of the lagrangian. In the case at hand, to reproduce the correct weak scale or equivalently the correct Z boson mass, the 3 remaining variables need to satisfy the constraint of the form
This constraint divides the mSUGRA parameter space into 3 regions:

If m_0 and M_1/2 are too small one cannot solve the above constraint for any μ. This corresponds to the "vev=0" region on the left-hand side of the plot.
For large SUSY breaking parameters the Higgs potential may not have a stable minimum. This corresponds to the "vev = ∞" region on the right-hand side of the plot.
In the remaining parameter space one can always choose μ such that the above constraint is satisfied. Nature could in principle choose one particular point in this region.

Unfortunately, most of this available parameter space has already been excluded by the LEP experiment back in the 90s. The failure to observe any superpartners of the Standard Model at LEP left only a narrow sliver of the parameter space close to the "vev=0". Now, the latest LHC results excluded a part of the remaining sliver, which is marked in the plot as the darker red region.

You may want to zoom in to fully appreciate the impact of the LHC searches:
Recall that blue is theoretically unavailable, light red is excluded by LEP, dark red is excluded by the LHC, and white is allowed. The breathtaking endeavor of the LHC for the next few years will be to further shrink the white stripe.

Of course, the way it is presented here is a bit tendentious, and in a larger picture things might look less bleak. For example, the plot refers to a very specific constrained SUSY scenario; in more favorable scenarios the unexcluded parameter space may be twice as large. Furthermore, the ATLAS and to some extent also the CMS search are not completely robust. Thus, one can easily design SUSY scenarios that are less constrained by the LHC, at least for another month until Moriond. For example, SUSY models without light charginos would be missed by the ATLAS search. As a last resort, one can always present the results such that the allowed parameter space is better visible:Here, the x-axis corresponds to a relative fine-tuning of a given point in the mSUGRA parameter space: small fine-tuning on the right (when SUSY parameters are or order the Z boson mass), big fine-tuning on the left. The allowed parameter space is the green chimney close to the left edge. The breathtaking endeavor of the LHC for the next few years will be to move the red region further up the chimney.

See also Alessandro's paper for more details. For more pedagogical and less malicious comments on LHC SUSY searches see this post on US/LHC Blog.

2 more years at 7

2011-01-31T18:13:00.007+01:00

It's official now. Here is an excerpt from DG's email:

...The main decisions we have taken are that the LHC will run through 2012 before a long shutdown, we'll keep the energy at 3.5 TeV during 2011, and we'll work hard to increase the luminosity steadily...

Seems reasonable enough to me. With the pressure from the Tevatron gone, it is not critical to squeeze every bit out of the machine. Hence the decision to remain at the 7 TeV center-of-mass energy of collisions which increases the chances that the thing will not explode in our hands again. More importantly, the run-1 is going to continue until the end of 2012. By that time, the LHC should have acquired some 5 inverse femtobarns of data, maybe more. This will be enough to see glimpses of new physics, provided there is anything below a TeV. But the most solid advantage of extending the run-1 is that the Higgs will be discovered earlier than in the alternative scenario with the 2012 shutdown. Thus, the minimum plan should be accomplished by the early 2013, if only Higgs is where we expect him to be. The disadvantage is that for 5 more years, rather than 4, I'll have to listen to talks about supersymmetry.

For a more illuminating analysis of the Higgs prospects, see Tommaso's blog.

Another Intriguing Result from Tevatron's CDF

2011-01-24T14:50:00.006+01:00

Two weeks ago we got excited about the forward-backward asymmetry of the top quark pair production. The CDF experiment says that the asymmetry at large t-tbar invariant mass is nearly 50%, as far as 3.4 sigma away from the standard model. A last week paper of Gilad Perez & co. points out that there exists another CDF measurement related to top quarks which also shows a large discrepancy from the standard model. The funny thing is that the latter result has been available as a public note since last summer but it went relatively unnoticed. The numbers and plots quoted below are take from this recent CDF talk.

The result I'm talking about is the CDF search for boosted tops. Here, "boosted" refers to top quarks wheezing out of the collision point with pT > 400 GeV. Such large pT implies that all the decay products of the top -- one b-quark + two quarks or two leptons from the W -- merge into a single object that looks very much like an ordinary QCD jet. Therefore CDF scanned their data for dijet events with at least one jet of pT > 400 GeV. The basic handle to distinguish boosted tops in that sample is the invariant mass of the jet: when the top quark decays fully hadronically (t → b + q + q) the mass of the jet parented by the top should be in the vicinity 170 GeV, whereas the masses of ordinary QCD jets span a wider range with a preference toward a lower value. The plot below shows the distribution of the masses of the high-pT jets as measured by CDF:
Although it is not immediately clear to the eye, there is an excess in the signal box. One counts 32 events in the top mass window, while QCD predicts only a third of that. It looks like CDF has captured 10-20 boosted tops. But the standard model predicts much less than that: only about 3 top events should be in the window! The excess is 3.4 sigma.

One could easily imagine new physics producing an excess of boosted tops; the canonical example would be a heavy gluon in the Randall-Sundrum scenario. Then why is CDF not jumping around claiming an evidence for new physics? The reason is that there is a certain tension with that interpretation of the signal. The fully hadronic decays, targeted by the above plot, constitute only a subset of the top events. Almost as often, one of the top quarks should decay leptonically, that is t → b + lepton + neutrino. For boosted tops, the charged lepton (e,μ,τ) merges with the b-jet and cannot be easily discerned, but the missing energy carried by the escaping neutrino should be registered. Thus, in the case of semileptonic decays of a boosted top pair one should observe one high-pT jet in the top mass window accompanied by large missing energy (those events were excluded from the previous plot). The relevant plot is pasted on the right. There is no excess, but actually a small deficit in the signal box where QCD predicts some 31 events and CDF observes 26. Because of the unclear interpretation, CDF decided to play it safe /swipe it under the carpet, chose one. In their note they dumped together the fully hadronic and semileptonic samples, in which case the excess becomes insignificant, and quoted only this unexciting in the abstract.

There are several possibilities on the table now. By far the most probable is that the hadronic excess is a fluke or is due to an underestimation of QCD background. Conversely, the deficit in the semileptonic channel could be a downward fluke, and there is indeed an excess of boosted tops at the Tevatron. The third possibility is to take all the hints from experiment seriously. An excess in the fully hadronic channel and the lack of an excess in the semileptonic channel may suggest that Tevatron is seeing a new particle that decays exclusively to hadrons and whose mass is accidentally close to the top quark mass. For example, the paper of Gilad & co. argues that a light gluino decaying into 3 quarks (that is assuming supersymmetry with R-parity violation) can be made consistent with the data. Whatever it is, this is a kind of signal that the LHC should eat for breakfast, if only it is real.

No Bosons for America

2011-01-10T21:53:00.014+01:00

Today Facebook and blogs are abuzz with the news that the operation of the Tevatron will not be extended beyond the financial year 2011. At first sight this may appear a short-sighted decision. If Tevatron continued until 2014 and doubled the luminosity acquired so far it would have a good chance to snatch the Higgs boson, possibly the biggest prize in particle physics in this century. So why backing down now? Why slaying a goose that is about to lay a golden egg? Of course, the real reason for closing the Tevatron is not the operation costs (peanuts) or the competition from the LHC (for a light Higgs, the Tevatron could get there first). The real reason is much more profound. The real reason is the fundamental law that I pointed out some time ago, which is known as Pauli's other exclusion principle:

Fermions are discovered in the US, whereas bosons are discovered in Europe.

This law has been tested in multiple instances, and has been established beyond all doubt. Evidently, the policy makers read blogs and are aware that any attempts to discover the Higgs boson at the Tevatron would be doomed from the start. Conversely, the DOE decision to shut down the Tevatron is yet another proof that Pauli's other exclusion principle is the fundamental law of nature that can never ever be violated.

New Physics for the New Year from CDF

2011-01-04T15:47:00.020+01:00

The year 2011 begins with an earthquake. Just yesterday I was speculating what good old Tevatron would bring as this year. The following morning we're awaken by trumpets and angel choirs announcing the new CDF paper. Inside it, the 2-sigma blip previously seen in the forward-backward asymmetry of the top quark production is promoted to a 3-sigma blip.

Tevatron collides protons with antiprotons, and the production of top-antitop pairs at the parton level is dominated by quark-antiquark collisions. Thus, one can define the forward direction along the proton beam (which is also the direction of the incoming quark) and the backward direction along the antiproton beam. One can then count the number of top quarks produced in the forward and backward directions, and similarly for antitop quarks. Both CDF and D0 studied this asymmetry in the Tevatron data, and found that top quarks prefer to go forward and the antitop quarks prefer to go backward. In the first approximation, there should not be any asymmetry in the standard model. However, loop corrections and jet radiation do in fact induce a small asymmetry of order a few percent. CDF and D0 were finding the asymmetry of the correct sign but a bit larger magnitude compared to the standard model prediction, with the discrepancy at the 2-sigma level. Intriguing, but not overly exciting.

Until today.

The new CDF paper updates their earlier analysis using 5.3 inverse femtobarns of the Tevatron data. It studies the semileptonic top events when one of the top/antitop quarks decays to b-quark + electron/muon + neutrino, and the other decays into 3 quarks. Measuring the momenta of all decay products one can reconstruct the original momenta of both tops, so that we know exactly (up to experimental uncertainties) in which directions they were produced. Furthermore, measuring the charge of the lepton identifies which one was the top and which one the antitop (the former decays to l+, the latter to l-). It is then trivial to count the difference of top quarks produced in the forward versus backward direction. Actually, the asymmetry depends a bit on the reference frame in which it is measured. CDF give the asymmetries measured in the lab frame and in the t-tbar rest frame, and they also unfold background contamination and resolution effects to obtain microwave-ready parton-level results. The numbers quoted in the following are parton level in the t-tbar frame, while the plot relates to the same frame before the unfolding.

The new key element in the CDF paper is that, thanks to improved statistics, they can study various kinematical distributions related to the asymmetry. In particular, they study how the asymmetry depends on the invariant mass of the t-tbar system (which tells you at what center of mass energy the pair was produced). While the inclusive asymmetry is small, of order few percent, the asymmetry at the high mass end of the spectrum appears to be huge, almost 50 percent. More precisely, CDF divides the t-tbar events into two groups, depending whether their invariant mass is smaller or larger than 450 GeV. In the former group the measured asymmetry is actually slightly negative, -12±15 %, perfectly consistent with the standard model prediction of 4 %. But for events with the invariant mass above 450 GeV the asymmetry is 48±11 %, as compared to 8% predicted by the standard model! The anomaly has a statistical significance of 3.4 standard deviations.

The fact that the asymmetry sharply grows with the invariant mass of the t-tbar system smells like a new heavy particle meddling into the top production process. The CDF paper tries out a heavy gluon with axial couplings to the light and the top quarks. The positive sign of the asymmetry (as observed) can be obtained assuming that the couplings to the light quarks has the opposite sign than that to the top quark. The model predicts roughly the right value and shape of the asymmetry for the gluon mass around 2 TeV (see the left plot). Of course, heavy gluon is not the unique possibility; other models have been proposed in the literature (Zprimes, color sextets, ...), all of them at least as ugly. The full spectrum of possibilities will be worked out in 123 papers due to appear on hep.ph by the end of the month.

As is the case with any anomaly, it is always more likely that the explanation is trivial. 3 sigma could well be a fluke. Also, some important physical contribution to the asymmetry may have been missed by theorists, so that the standard model prediction has been grossly underestimated. Another possibility is that CDF observed a fundon (an elementary particle produced in high-energy colliders near the end of the budgetary cycle); forward-backward asymmetry is one type of measurement where Tevatron is superior to the LHC (whose initial state is p-p, rather than p-pbar, which obscures this kind of analysis). On the other hand, if a colored particle with a TeV mass is responsible for the asymmetry, finding the particle should be a piece of cake for the LHC. Theory error, fundon, or KK gluon...time will tell. Soon.

The Year of Living Dangerously

2011-01-03T13:57:00.009+01:00

The previous 4 years of Resonaances were but a constant decay. As you can read from the plot of my posting activity versus time, the end is expected in late 2012. The end of the blog, or the of the world, don't know which. Yet before this happens we have the year 2011 that, for obvious reason, is going to provide us with more excitement than the entire last decade. Here is what I'm waiting for the most (ordered according to the level of my impatience to see the results):

LHC
It's the year of truth for particle physics. If there exist new colored particles below TeV (expected in basically any theory that addresses the hierarchy problem of the standard model) we should catch at least a glimpse of them this year. If nothing shows up...well, we can always dream of new physics behind the next corner, but we better start telling people that the LHC has always been about the Higgs :-)
Xenon100
The most sensitive dark matter detection experiment is now sitting on almost 150 days of unpublished data, in addition to the small bone of 11 days they threw us in early 2010. The new results should be published some time soon, although Xenon100 does not commit to a specific date. The prospective limits on dark matter - nucleon cross section should not be too far from the dotted red line on the plot (which assumes 200 days)....unless the signal is there. If not this year then when?
Tevatron
It's probably the final year of running for this machine. At this stage it is unlikely that new physics will jump in our faces, unless it is concealed in some truly exotic channel that nobody has thought to look at so far. Nevertheless, the Tevatron still has a potential to provide some excitement this year. First of all, there will surely be new Higgs limits this summer: another chunk of Higgs masses will be crossed out, and maybe we'll even see a 2-sigma hint of the real thing. On the new physics front, we are waiting for updates on CP violation in the B-meson system, as this is the place where the most intriguing anomalies have showed up. And, who knows, maybe some other 2-sigma blip will be promoted to a 3-sigma blip, the good candidates being the forward-backward top-quark production asymmetry, the t-prime hunt, or the Higgs partner search in the 3-b channel.
IceCube
This neutrino telescope, named in honor of the gansta rapper, has just been completed last month. IceCube is a chunk of ice at the south pole where high energy muon neutrinos are invited to convert into muons, whose Cherenkov radiation is then detected by strings of photomultipliers embedded in the ice. These new eyes on the universe will surely bring a fascinating progress into conventional astrophysics. Can there be hints of new physics in cosmic ray neutrinos, or in neutrinos from the Sun? Why not.
MEG
This one could be a dark horse, although it's not even clear if they will be joining the race this year. MEG is another experiment with muons at PSI, Switzerland. It is looking for the μ →e γ decay whose rate is unobservable in the Standard Model, but could be easily enhanced in many of its extensions. Last summer at ICHEP MEG presented the results of an early 2-month run that showed an intriguing clustering of events near the signal region (although none of the events passes the timing + angle cuts). They're planning to continue running until the end 2012 so as to bring the sensitivity down to 10^-13 branching fraction. But if the events keep popping up we may hear about them sooner.
Up in the Sky
This year it will be more quiet up there . The AMS-II experiment is going to be launched to the ISS early this year (unless they decide to replace their magnet with a refrigerator magnet) but we'll have to wait years for any interesting results. PAMELA and Fermi are still up there and we may still learn something new about the cosmic ray spectrum. But, most likely, the sky won't rock before 2012 when Planck publishes their first CMB analysis.

If i forgot something to be impatient about, don't hesitate to point it out.

2010 Highlights

2010-12-31T18:47:00.006+01:00

It's the end of the year when blogosphere and old-fashioned press alike indulge in a nostalgic mood. Here is my list of the most exciting events of the passing year in the field of particle physics. From the year 2010 I remember (in chronological order):

CoGeNT, for making us drunk with light dark matter.
This experiment created the largest stir in theory this year. CoGeNT, a dark matter detection experiment, announced that it could be seeing dark matter with a relatively light mass, around 10 GeV. The dominating paradigm is dark matter at the weak scale, 100 GeV to 1 TeV, but the CoGeNT result made us stop and think about a wider range theoretical possibilities. Unfortunately, recent exclusion limits results from Xenon10, Xenon100 and CDMS make it highly unlikely that CoGeNT is really observing dark matter. Nevertheless, the lesson we have learned is that dark matter does not have to be where everyone is looking.
D0, for keeping the hopes for new physics alive.
Good old Tevatron gave us one very intriguing result this year. The D0 collaboration looked into same-sign di-muon events, and found that events with two negative muons occur 1 percent more often that those with two positive muons. This result can be interpreted as CP violation in the B-meson system: the Bbar-mesons oscillate into B-mesons a bit more often than the other way around. The Standard Model predicts such an effect, but the asymmetry should be 100 times smaller that what is observed. Is this new particles contributing to the B-meson mixing? Or did D0 screw up? The jury is still out.
PSI, for extending the new physics battlefield into atom spectroscopy.
The surprise of the year, no doubt about it. A laser spectroscopy experiment at PSI measured the Lamb shift in muonic hydrogen, and found it to be 5 sigma away from the prediction based on theory and earlier experiments with ordinary hydrogen. Given that simple new physics models cannot provide a consistent explanation, and that QED is doing shamelessly well everywhere else, we all expect that some theoretical or experimental error is at the root of this anomaly. But the possibility that some quirky new physics manifests itself here is still hanging in the air.
Tevatron, for its tireless Higgs chase.
Tevatron gave us also a completely expected yet very cute result. 10 years ago LEP excluded the Higgs masses below 115 GeV, now Tevatron tells us that Higgs between 156-175 GeV is not the right answer either. Combining that with precision electroweak tests, we deduce that Higgs is cowardly hiding somewhere between 115 and 155 GeV. Poor bastard is thus cornered, and with the LHC joining in the chase he should surrender in no time. Unless he is not there after all...
LHC, for the overall impression.
After a series of setbacks and delays this year LHC surprised us, for a change, with a stream of good news. We had been told that the first year would be a total mess, as it should take a long time to understand the detectors enough to produce meaningful results. Instead, physics results have been delivered basically from day 1, even in difficult channels like jets + missing energy. LHC already published several important limits, e.g. on 4-quark operators (gracefully called "bounds on compositeness"), or on high-energy high-multiplicity events (under the sexy name of "limits on black hole production"). And much more is due to arrive for the winter conferences. It's easy to predict that the LHC will make it to the 2011 highlights on Resonaances; the only question is whether I will remember it for "important limits" again, or for crazy new discoveries...

Is the CKM matrix going to crack?

2010-12-23T02:25:00.007+01:00

During the last decade the Standard Model description of flavor transitions has been put to multiple tests, especially in the B-meson sector. The overall agreement between theory and experiment is excellent, much better than what we should expect assuming exotic particles lurking just behind the corner. Here and there, however, one finds a few glitches - most likely experimental flukes or underestimated theory errors but intriguing enough to keep a flicker of hope alive. This year there has been a lot of commotion about the D0 observation of the same sign di-muon asymmetry, since the Standard Model predicts this effect should be well below the current experimental precision. If the D0 result is confirmed, it would be a clear indication of new physics contribution to CP violation in the mixing of neutral B-mesons. Another, less publicized 3-sigma blip is the tension between:

the CP asymmetry in the Bd meson decay into J/ψ + kaon,
the branching fraction of the decay of a charged B meson into a tau lepton and a tau neutrino.

This tension has been around for a while, but below I'll follow a more recent presentation by Lunghi and Soni who put a slightly different twist to it.

All this fuzz is about measuring the entries of the CKM matrix - a 3x3 unitary matrix that is the source of all flavor violation in the Standard Model. See the usual parametrization pasted on the right. The parameters λ and A are well measured in several different ways that yield consistent results. Therefore one is more interested in constraints on the remaining two parameters called ρ and η. The 2 processes mentioned in the previous paragraph are sensitive to slightly different combinations of these parameters. The B → τν decay proceeds at tree-level via an off-shell W-boson, so the branching fraction is proportional to the Vub, that is the (13) element. Thus, the measurement of this branching fraction carves out a circle in the ρ,η plane. On the other hand, the CP asymmetry Bd → J/ψ K is due to an interference of tree-level decays and one-loop B-meson mixing, and the final result depends on Sin(2β) where β ∼ Arg[Vtb Vtd/Vcs Vcb ] is one of the angles in the unitarity triangle. This measurement appears as a diagonal line in the ρ,η plane. Now let us see how these two processes combine with several other measurements of ρ and η:
The point is that one can reconcile either of the two measurements with the other constraints on ρ,η but accommodating both is difficult. For example, in the upper plot B → τν is included in the fit to ρ and η. That best fit value uniquely predicts Sin(2β), but the result is off from the experimental value by more than 3 sigma. Conversely, if one uses Bd → J/ψ K in the fit, then B → τν is off by almost 3 sigma. The authors prefer the former interpretation because it provides a better overall consistency of the fit. This interpretation is also more plausible from the new physics point of view, since in general it is easier for new physics to compete with Standard Model loop processes than with tree-level processes. Moreover, this way it may go along better with the D0 di-muon anomaly as the latter is also related to B-meson mixing...

Now, how large is the tension clearly depends on the choice of observables going into the fit,
as well as on your personal beliefs in the errors quoted by various theoretical, experimental and lattice groups whose results enter the fit. For example, in the similar plots presented by the CKMfitter collaboration the errors are more conservative and the tension is not apparent. Clearly, on tabloid blogs such as Resonaances the aggressive approach is promoted, but one should remember that the cautious approach to flavor anomalies is usually right, at least historically. Asymptotically in the future, the new generation of B-factories (who should go online in late two-thousand-teens) will shrink the errors and swipe the floor. In a shorter time perspective, updates from Tevatron may clarify or further blur the situation. And then we're dying to see LHCb joining in the game, some time next year. But the last one is a perfect subject for a separate post...

Update on Muonic Hydrogen

2010-12-03T00:44:00.010+01:00

5 months ago an experimental group at PSI announced the measurement of the Lamb shift in muonic hydrogen. Since muon is 200 times heavier than electron, the muonic hydrogen atom is 200 times smaller than the ordinary hydrogen. Therefore, finite proton size effects are far more pronounced in the former, and end up contributing as much as 2 percent to the Lamb shift. Assuming that the system is adequately described by QED, the PSI result can be interpreted as a new measurement of the size (charge radius) of the proton. The surprise was the deduced value of the charged radius turned out to be inconsistent at the 5 sigma level with the previous determinations based on the spectroscopy of hydrogen and electron-proton scattering data. Something is wrong. Either there is an experimental error, or there is an error in the theoretical computations of the Lamb shift, or maybe some new forces are in the game.

Of course, it is the last of the above possibilities that makes the anomaly attractive to hoards of hungry-eyed particle theorists. In fact, it's not the first mysterious result related to the muon: a 3-point-something-sigma anomaly in the muon anomalous magnetic moment has been nudging us for years. It is tempting to speculate that both these muon anomalies have a common explanation in terms of yet unknown fundamental forces. Furthermore, as I explained here, new hidden forces have recently become very popular in the particle circles for other, completely unrelated reasons. Yet ArXiv has not been flooded with theory papers on muonic hydrogen, so far. The reason is that it's difficult to write down a new physics model that explains the measured Lamb shift without violating constraints from atomic precision physics. The most painful constraints come from

Ordinary hydrogen spectroscopy,
Anomalous magnetic moment of the electron,
Low energy neutron scattering experiments,
Interactions of neutrinos with matter.

These constraint exclude popular models, such as the hidden photon or a new Higgs-like scalar, as the explanation of the anomaly. One could thus conclude that there is nothing interesting here and move on. Or one could promote the positive attitude. Like in this paper last week by David Tucker-Smith and Itay Yavin who, apart from mounting difficulties, also proposes a solution.

The paper proposes how to shift the energy levels of muonic hydrogen without violating other experimental constraints. The first part is easy: a scalar or vector particle could provide for the new attractive force that does the job. One possibility is to take the mass of the new particle to be of order MeV, and the coupling to muons and protons of order $10^{-4}$ (the contribution to the Lamb shift scales as $g_\mu g_p/m^2$ for m above 1 MeV and $g_\mu g_p m^2/m_\mu^4$ for m less than 1 MeV; thus other choices of the parameters are possible, for example, for a larger mass one would need correspondingly larger couplings). With the couplings and the mass in the same ballpark one could also obtain a new contribution to the muon anomalous magnetic moment that resolves the tension with experiment, see the blue band in the plot.

Now comes the tricky part, that is addressing other experimental constraints. There are some older muonic-atom experiments, for example the one with Mg and Si, who constrain the couplings of new force carriers to muons and protons. However, they are not inconsistent with the coupling strength needed to explain the muonic hydrogen anomaly. But it seems the new force carrier has to couple only to muons and protons and virtually nothing else. For example, the coupling to electrons has to be at least an order of magnitude smaller than that to muons in order to avoid excessive contributions to the anomalous magnetic moment of the electron. The coupling to neutrons is even more strongly constrained by some prehistoric experiments (from 1966!!! back when England last won the world cup!!! ;-) involving low energy neutrons scattering on lead atoms. Furthermore, B-factories strongly constrain the couplings to b-quarks, neutrino experiments strongly constrain the couplings to neutrinos, and so on.

It is simple to cook up a model where the coupling of the new force carrier to electrons is suppressed (a particle coupled to mass), or when the coupling to neutrons is suppressed (a particle coupled to charge), but to achieve both at the same time is a model-building challenge. However this possibility cannot be excluded in a model independent manner, so it open to experimental verification. If a new force carrier is the reason for the muonic anomalies, there should be shifts in the spectrum of other muon systems, such as muonic helium or the true muonium (a bound state of muon and antimuon). Those systems have not been investigated yet, but with the present technology they seem to be within reach. So, if you have some free time this weekend you could try to make the true muonium and measure its energy levels. Depending on the result, life could get very interesting, or it could get as usual...

See also here and here to better appreciate the problems with model building. For a fresh review and reevaluation of the standard QED contributions to the muonic hydrogen energy levels, see here.

Wooden Stake for CoGeNT

2010-11-15T03:30:00.013+01:00

There is a new interesting paper from CDMS that excludes an important region of the parameter space of dark matter models.

First, a short summary of previous episodes. Earlier this year CoGeNT made a claim of possible detection of dark matter. CoGeNT is a relatively small dark matter experiment using a germanium detector located in the Soudan mine. The spectrum of events they registered during the first months of operation is consistent with scattering of dark matter particles with the mass of order 10 GeV and the cross section on nucleons of order 10^-40cm2. Dark matter in this mass ballpark could also fit 1) the long-standing DAMA modulation signal, 2) the 2 events observed by CDMS last year, and 3) the oxygen band excess reported by the CRESST experiment. These developments came somewhat unexpected to most of us, as the dominant theoretical prejudice would place dark matter at a somewhat heavier scale, 100 GeV or so. Following this prejudice, the majority of dark matter experiments were optimizing their search strategies for the weak scale dark matter, neglecting the light mass region. The typical recoil resulting from a 10 GeV particle scattering in a detector would be too small to pass the threshold set by most experiments. The advantage of CoGeNT is a very low energy threshold, 0.4 keV in ionization energy translating to about 2 keV true recoil energy. This is the key reason why they could achieve a better sensitivity for light dark matter than the big guys in the detection business such as the CDMS and Xenon collaborations whose analysis threshold had been higher.

Nevertheless, the big guys didn't despair, but have worked toward improving sensitivity in the CoGeNT region. First came Xenon100. Using their early data they were able exclude the region of the parameter space consistent with the CoGeNT signal. The precise extent of their exclusion region is however controversial, because it strongly depends on poorly measured scintillation efficiency in xenon at low recoil energies, the so-called Leff parameter. Using more conservative assumptions about Leff, some of the CoGeNT parameter space remains allowed. Furthermore, the limits on light dark matter critically depend on certain unknown properties of dark matter, such as its velocity distribution in our galaxy; changing some assumptions could result in an enhanced event rate in a germaniun detector as compared to a xenon detector. For all these reasons, the Xenon100 exclusion was not considered conclusive.

Now the situation is clarified when the CDMS collaboration has recycled their old data so as to improve sensitivity for light dark matter. They lowered their recoil energy threshold down to 2 keV (as compared to 10 keV in their previous analysis). Lowering the threshold comes with the price, as at such low recoil CDMS cannot use the phonon timing cuts to better differentiate nuclear recoils (expected from dark matter scattering events) from electron recoils produced by all sorts of pesky backgrounds. The discriminating variable that remains available is the ionization yield (nuclear recoils typically produce small ionization, in a well-defined band) but that is not enough to get rid all of the events, see the plot above. Thus, whereas previous CDMS searches were expecting less than 1 background event, the new analysis has to deal with hundreds. Still, the dark matter cross section on nucleons that would be consistent with the CoGeNT signal would produce many more events than CDMS has observed. Assuming that all observed events come from dark matter (this is very conservative, as they are able to assign these events to known sources, such as surface events or noise) allows them to set pretty tight limits on the cross section in the low mass, see the solid black line in the upper plot. The CoGeNT region (shaded blue) is now comfortably excluded.

CDMS uses the the same germanium target as CoGeNT, so even theorists may find it hard to come with an explanation how dark matter could produce a signal in one and not in the other. Therefore it seems safe to pronounce the CoGeNT signal dead. Too bad. However the dark matter detection business regularly produces new entertainment; maybe the much-expected soon-to-come one-year Xenon100 results will provide us some?

The die has been RECAST

2010-10-25T22:41:00.008+01:00

RECAST is an idea toward a more efficient use of experimental data collected by particle physics experiments. A paper outlining the proposal appeared on ArXiv 2 weeks ago. In order to explain what RECAST is and why it is good I need to make a small detour.

In the best of all worlds, all experimental data acquired by humanity would be stored in a convenient format and could be freely accessed by everyone. Believe it or not, the field of astrophysics is not so far from this utopia. The policy of the biggest sponsors in that field - NASA and ESA - is to require that more-or-less data (sometimes a pre-processed form) are posted some time, typically 1-2 years, after the experiment starts. This policy is followed by such cutting-edge experiments as WMAP, FERMI, or, in the near future, Planck. And it is not a futile gesture: quite a few people from outside of these collaborations have made a good use these publicly available data, and more than once maverick researchers have made important contributions to physics.

Although the above open-access approach appears successful, it is not being extended to other areas of fundamental research. There is a general consensus that in particle physics an open-access approach could not work because:

bla bla bla,
tra ta ta tra ta ta,
chirp chirp,
no way.

Consequently, data acquired by particle physics collaborations are classified and never become available on the outside of the collaboration. However, our past experience suggests that some policy shift might be in order. Take for example the case of the LEP experiment. Back in the 90s the bulk of experimental analyses was narrowly focused on a limited set of models, and it is often difficult or impossible to deduce how these analyses constrain more general models. One disturbing consequence is that up to this day we don't know for sure whether the Higgs boson was beyond LEP's reach or whether it was missed because it has unexpected properties. After LEP's shutdown, new theoretical developments suggested new possible Higgs signatures that were never analyzed by the LEP collaborations. But now, after 10 years, accessing the old LEP data requires extensive archeological excavations that few are willing to undertake, and in consequence scores of valuable information are rotting in the CERN basements. The situation does not appear to be much better at the Tevatron where the full potential of the collected data has not been explored, and it may never be, either because of theoretical prejudices, or simply because of lack of manpower within the collaborations. Now, what will happen at the LHC? It may well be that new physics will come straight in our faces, and there will never be any doubt what the underlying model is and what are the signals we should analyze. But it may not... Therefore, it would be wise to organize the data such that they could be easily accessed and tested against multiple theoretical interpretations. Since an open access is not realistic at the moment, we would welcome another idea.

Enter RECAST, a semi-automated framework enabling to recycle existing analyses so as to test for alternative signals. The idea goes as follows. Imagine that a collaboration performs a search for a fancy new physics model. In practice, what is searched for is a set of final states particles, say, a pair of muons, jets with unbalanced transverse energy, etc. The same final state may arise in a large class of models, many of which the experimenters would not think of, or which might not even exist at the time the analysis is done. The idea of RECAST is to provide an interface via which theorists or other experimentalists could submit a new signal (simply at the partonic level, in some common Les Houches format). RECAST would run the new signal through the analysis chain, including hadronization, detector simulations and exactly the same kinematical cuts as in the original analysis. Typically, most experimental effort goes into simulating the standard model background, which has already been done by the original analysis. Thus, simulating the new signal and producing limits on the production cross section of the new model would be a matter of seconds. At the same time, the impact of the original analyses could be tremendously expanded.

There is some hope that RECAST may click with experimentalists. First of all, it does not put a lot of additional burden on collaborations. For a given analysis, it only requires a one-time effort of interfacing it into RECAST (and one could imagine that at some point this step could be automatized too). The returns for this additional work would be a higher exposure of the analysis, which means more citations, which means more fame, more job offers, more money, more women... At the same time, RECAST ensures that no infidel hands ever touch the raw data. Finally, RECAST is not designed as a discovery tool, so the collaborations would keep the monopoly on that most profitable part of the business. All in all, lots of profits for a small price. Will it be enough to overcome the inertia? For the moment the only analysis available in the RECAST format is the search for Higgs decaying into 4 tau leptons performed recently by the ALEPH collaboration. For the program to kick off more analyses have to be incorporated. That depends on you....

Come visit the RECAST web page and tell the authors what you think about their proposal. See also another report, more in a this-will-never-happen vein.

Maybe all that exists is the standard model...or even less

2010-10-18T00:08:00.007+01:00

Throughout the previous decade Gia Dvali was arguing that there are $10^{32}$ copies of the standard model out there. Now, he made a U-turn and says that there is only 1. Or even less. Let me explain.

The reason why we are pretty sure that we are going to observe new phenomena in the LHC goes under the nickname unitarity of WW scattering. What hides behind this is, technically speakin, that the tree-level scattering amplitude of longitudinally polarized W bosons computed in the standard model without the Higgs particle grows as a square of the scattering energy, and at some point around 1 TeV it becomes inconsistent with unitarity, that is with conservation of probability. In the full standard model this problem is cured: the contribution from the Higgs exchange cancels the dangerously growing terms and the full amplitude is well behaving for arbitrary high energies. A slightly different mechanism is realized in technicolor theories, where the consistent UV behavior of the amplitude is ensured by the exchange of spin-1 resonances.
In spite of 40 years of intensive research we are only aware of these 2 ways of unitarizing the WW amplitude. Thus the LHC should see either the Higgs or new spin-1 resonances. Time will tell which of the 2 possibilities is realized in nature.

A paper last week by Dvali and co. suggests that there may be a 3rd possibility. The authors conjecture that the standard model without a Higgs and without any other embellishments could be a fully consistent theory, even though it appears to be in conflict with unitarity. They argue that the uncontrolled growth of the WW scattering amplitude is just an artifact of the perturbative approximation, while at the non-perturbative level the theory could be completely sane. The idea is that, as the scattering energy increases above TeV, the theory defends itself by producing "large" classical configurations during the scattering process. The higher the energy, we get the larger and more classical objects which then decay preferentially to many-body (rather than 2-body) final states. This way the 2-to-2 WW scattering remains unitary at energies above TeV. The authors, somewhat dully, call this mechanism classicalization. To put it differently, as we increase the scattering energy at some point we stop probing the physics at short distance scales; these small distances are screened from external observers, similar in spirit to black holes screening the short distance physics in transplanckian scattering when gravity is in the game.

If this is the case, what would it mean in practice, that is in experiment? Much as in technicolor, at TeV energies the LHC should observe resonances in WW scattering who ensure the unitarity of the perturbative amplitude in the low-energy effective theory. However, as the scattering energy is increased the resonances become more and more classical and spectacularly decay into many-particle final states. There is no new fundamental degrees of freedom at high energies, no new fundamental forces to discover, just the standard model and its non-perturbative classical dynamics.

Now, can this be true? The paper is rather cryptic, and provides few technical details. In this sense it feels like another emergent gravity. What it demonstrates is that in a class of theories that includes the standard model there exist classical solutions whose large distance behavior only depends on how much energy is sourcing it, and whose size grows in a universal way with the energy. The rest seems to be just words, and there is a long way to proving that classicalization can indeed lead to a fully consistent quantum theory. Nevertheless, given the scarcity of ideas concerning electroweak symmetry breaking, there is definitely some philosophical potential in the paper. We'll see whether it leads to something more concrete...

Update: See also Lubos' stance.

Back in Town

2010-10-16T14:23:00.005+01:00

More than 2 months have passed since my last post. Sorry for these perturbations related to changing continents. I'm about to resume blogging after a few changes and adaptations due to my new environment:

One is the cute new banner you must have seen already.
Furthermore, the name of this blog has been changed from Resonaances to Résonaances.
Seriously ;-)
All you fellow bloggers, you should update the name in your blog roll, otherwise you risk being sued by La Commission de la Protection de la Langue Française.
Mhm, I actually found out that La Comission does not exist anymore, but one never knows...
And all you readers, mind that the pronunciation has changed :-)
The subsequent posts will have an abstract in French.
Joking, of course. French is perfect for flirting, but not as much for talking science.
Consequently, author's name remains Jester, it has *NOT* been changed to Le Bouffon ;-)

The last two months when I was out have been quiet anyway. Dark matter was discovered, again. Higgs was rumored to be seen at the Tevatron, again. Some unexplained events have been seen at the LHC. Just business as usual. These latter rumors should be exponentially growing with each picobarn acquired by the LHC; in case, you know where to ask ;-)

His First Inverse Picobarn

2010-08-09T11:18:00.010+01:00

The LHC, more precisely ATLAS, just passed the 1pb-1 milestone:

One inverse picobarn of integrated luminosity is 1/1000 of what is planned for LHC Run I. At the 7 TeV center-of-mass energy this luminosity translates to:

200 000 W bosons,
60 000 Z bosons,
200 top quark pairs,
10-20 Higgs bosons, if bastard's mass is around 120 GeV,
A couple of gluino pairs (Poisson permitting) in a parallel universe where gluinos exist and weigh 500 GeV.

It's...

2010-08-05T16:51:00.008+01:00

A new paper entitled It's on is now out on hep-ph. When particle theorists refer to "it" they don't mean sex, unlike ordinary people. Here "it" stands for the LHC who is not only "on" but already produces interesting constraints on new physics. In particular, the latest jets + missing energy search performed by ATLAS excludes a new region of the susy parameter space with a light, 150-300 GeV gluino. One can learn two interesting things from the paper:

1) that with just 70 nb-1 of LHC data one can obtain non-trivial constraints on vanilla susy models that in some cases are more stringent than the existing Tevatron constraints,
2) and that the susy combat group in ATLAS missed the point 1.

A gluino is the fermionic partner of the QCD gluon, as predicted by supersymmetry. A pair of gluinos can be produced for example by colliding 2 gluons. Since the protons circulating in the LHC ring are filled to the brim with gluons, gluinos would pop out as pop corn if only they existed and were light enough. For example, a 200 GeV gluino would be produced at the LHC7 with the stunning cross section of 0.6 nb. Thus, even the small amount of LHC data collected so far could contain a few tens of gluino events. Once produced, gluinos immediately decay to standard model and other susy particles (if they don't then the whole story is completely different, and is not covered by the latest ATLAS search). When the gluino is the next-to-lightest susy particle apart from a neutralino then it decays via an off-shell squark into 2 quarks and the neutralino. The signature at the LHC is thus a number of high-pT QCD jets (from the quarks) and large missing energy (from the neutralinos who escape the detector).

There is a lot of jet events at the LHC, but fortunately only a small fraction of them is accompanied by large missing energy. In the 70nb-1 of data, after requiring 40 GeV of missing pT, and with some additional cuts on the jets one finds only four such dijet events, zero 3-jet events, and one 4-jet event. Thus, even a small number of gluinos would have stood out in this sample. The resulting constraints on the gluino vs. neutralino masses are plotted below (the solid black line)
In the region where the mass difference between the gluino and the neutralino is not too large, the LHC constraints beat those from the Tevatron, even though the latter are based on 100000 more luminosity! Obviously, the constraints will get much better soon, as the LHC has already collected almost 10 times more luminosity and doubles the data sample every week.

These interesting constraints were not derived in the original experimental note from ATLAS. Paradoxically, many experimentalists are not enthusiastic about the idea of interpreting the results of collider searches in terms of directly observable parameters such as masses and cross sections. Instead, they prefer dealing with abstract parameters of poorly motivated theoretical constructions such as mSUGRA. In mSUGRA one makes a guess about the masses of supersymmetric particle at the scale $10^{14}$ times higher then the scale at which the experiment is performed, and from that input one computes the masses at low energies. The particular mSUGRA assumptions imply a large mass difference between the gluino and the lightest neutralino at the weak scale. In this narrow strip of parameter space the existing Tevatron searches happen to be more sensitive for the time being.

CoGeNT dark matter excluded

2010-07-31T15:10:00.004+01:00

They say that il n'y a que Paris. This is roughly true, however Paris last week was not the best place in France to learn about the latest dark matter news. Simultaneously to ICHEP'10 in Paris down south in Montpellier there was the IDM conference where most of the dark matter community was present. One especially interesting result presented there concerns the hunt for light dark matter particles.

Some time ago the CoGeNT experiment noted that the events observed in their detector are consistent with scattering of dark matter particles of mass 5-10 GeV. Although CoGeNT could not exclude that they are background, the dark matter interpretation was tantalizing because the same dark matter particle can also fit (with a bit of stretching) the DAMA modulation signal and the oxygen band excess from CRESST.

The possibility that dark matter particles could be so light caught experimenters with their trousers down. Most current experiments are designed to achieve the best sensitivity in the 100 GeV - 1 TeV ballpark, because of prejudices (weak scale supersymmetry) and some theoretical arguments (the WIMP miracle), even though certain theoretical frameworks (e.g asymmetric dark matter) predict dark matter sitting at a few GeV. In the low mass region the sensitivity of current techniques rapidly decreases. For example, experiments with xenon targets detect scintillation (S1) and ionization (S2) signals generated by particles scattering in a detector. Measuring both S1 and S2 ensures very good background rejection, however the scintillation signal is the main showstopper to lowering the detection threshold. Light dark matter particles can give only a tiny push to much heavier xenon atoms, and the experiment is able to collect only a few, if any, resulting scintillation photons. On top of that, the precise number of photons produced at low recoils (described by the notorious Leff parameter) is poorly known, and the subject is currently fiercely debated with knives, guns, and replies-to-comments-on-rebuttals.

It turns out that this debate may soon be obsolete. Peter Sorensen in his talk at IDM argues that xenon experiments can be far more sensitive to light dark matter than previously thought. The idea is to drop the S1 discrimination, and use only the ionization signal. This allows one to lower the detection threshold down to ~1 keVr (while it's order 10 times higher when S1 is include) and gain sensitivity to light dark matter. Of course, dropping S1 also increases background. Nevertheless, thanks to self-shielding, the number of events in the center of the detector (blue triangles on the plot above) is small enough to allow for setting strong limits. Indeed, using just 12.5 day of aged Xenon10 data a preliminary analysis shows that one can improve on existing limits for the dark-matter-nucleon scattering cross section in the 5-10 GeV mass interval:Most interestingly, the region explaining the CoGeNT signal (within red boundaries) seems by far excluded. Hopefully, the bigger and more powerful Xenon100 experiment will soon be able to set even more stringent limits. Unless, of course, they will find something there...

Higgs still at large

2010-07-26T17:27:00.004+01:00

Finally, the picture we were dying to see:
Tevatron now excludes the standard model Higgs for masses between 156 and 175 GeV. The exclusion window widened considerably since the last combination. Together with the input from direct Higgs searches at LEP and from electroweak precision observables it means that Higgs is most likely hiding somewhere between 115 and 155 GeV (assuming Higgs exists and has standard model properties). We'll get you bastard, sooner or later.

One interesting detail: Tevatron can now exclude a very light standard model Higgs, below 110 GeV. Just in case LEP screwed ;-) Hopefully, Tevatron will soon start tightening the window from the low mass side.

Another potentially interesting detail: there is some excess of events in the $b \bar b$ channel where a light Higgs could possibly show up. The distribution of the signal-to-background likelihood variable (which is some inexplicably complicated function that mortals cannot interpret) has 5 events in one of the higher s/b bins, whereas only 0.8 are expected. This cannot be readily interpreted as the standard model Higgs signal, as this should also produce events with higher s/b where there is none. Most likely the excess is a fluke, or maybe some problem with background modeling. But it could also be an indication that something weird is going on that does not quite fit the standard model Higgs paradigm. Maybe the upcoming Tevatron publications will provide us with more information.

More details in the slides of the ICHEP'10 talk by Ben Kilminster.

Monday at ICHEP

2010-07-25T10:24:00.005+01:00

This Monday at ICHEP there will be a plenary talk by Nicolas Sarkozy. Like all theorists I'm looking forward to it, as he knows about models much more than we do. You can watch the webcast here, at high noon Paris time.

D0 says: neither dead nor alive

2010-07-24T16:29:00.003+01:00

This year CP violation in the Bs meson system has made the news, including BBC News and American Gardener. The D0 measurement of the same-sign dimuon asymmetry in B decays got by far the largest publicity. Recall that Tevatron's D0 reported 1 percent asymmetry at the 3.1 sigma confidence level, whereas the standard model predicts a much smaller value. The results suggests a new source of CP violation, perhaps new heavy particles that we could later discover at the LHC.

The dimuon asymmetry is not the only observable sensitive to CP violation in the Bs system. Another accessible observable is the CP violating phase in time-dependent Bs decays into the J/ψ φ final state. In principle, the dimuons and J/ψ φ are 2 different measurements that do not have to be correlated. But there are theoretical arguments (though not completely bullet-proof) that a large deviation from the standard model in one should imply a large deviation in the other. This is the case, in particular, if new physics enters via a phase in the dispersive part of the Bs-Bsbar mixing amplitude ($M_{12}$, as opposed to the absorptive part $\Gamma_{12}$), which is theoretically expected if the new particles contributing to that amplitude are heavy. The previous, 2-years old combination of the CDF and D0 measurements displayed an intriguing 2.1 sigma discrepancy with the standard model. CDF updated their result 2 months ago and, disappointingly, the new results is perfectly consistent with the standard model. D0 revealed their update today in an overcrowded room at ICHEP. Here is their new fit to the CP violating phase vs. the width difference of the 2 Bs mass eigenstates
Basically, D0 sees the same 1.5 sigmish discrepancy with the standard model as before. Despite 2 times larger statistics, the discrepancy is neither going away nor decreasing, leaving us children in the dark. Time will tell whether D0 found hints of new sources of CP violation in nature,
or merely hints of complicated systematical effects in their detector.

European Tops at Last!

2010-07-23T22:29:00.006+01:00

Today at ICHEP CMS and ATLAS showed their first top candidate events. They see events in both semileptonic and dileptonic channels, with muons and electrons in all combination. Here is one event display in the mu+jets+missing energy channel provided by CMS:

The reconstructed top mass from this event is around 210 GeV, while the latest measurement of the top quark mass from the Tevatron is 173.1 GeV. This is very surprising - naively, one would expect the American top quarks to be heavier ;-)

See more events from Atlas and CMS.

Working for a Paycheck

2010-07-21T09:40:00.002+01:00

ICHEP'10 is starting tomorrow in Paris. As I told you the other day, I was hired to blog on the highlights of the conference. So for the entire next week I'm planning to scribble a couple of posts per day - an unusual and probably lethal frequency for a lazy blogger accustomed to writing once a month. I guess I will copy&paste the most interesting posts here to Resonaances, but if you're interested in my entire discography you should check out the official ICHEP blog. A bunch of other good fellows writing there, so should be fun.

Muonic Hydrogen and Dark Forces

2010-07-16T06:19:00.009+01:00

The measurement of the Lamb shift in the muonic hydrogen has echoed on blogs and elsewhere. Briefly, an experiment at the Paul Scherrer Institute (PSI) measured the energy difference between 2S(1/2) and 2P(3/2) energy levels of an atom consisting of a muon orbiting a proton. Originally, this excercise was intended as a precise determination of the charge radius (that is the size) of the proton: in the muonic hydrogen the finite proton size effect can shift certain energy levels by order one percent, much more than in the ordinary hydrogen, while other contributions to the energy levels are quite precisely known from theory. Indeed, the PSI measurement of the proton charge radius is 10 times more precise than previous measurements based on the Lamb shift in the ordinary hydrogen and on low-energy electron-proton scattering data. Intriguingly, the new result is inconsistent with the previous average at the 5 sigma level.

As usual, when an experimental result is inconsistent with the standard model prediction the most likely explanation is an experimental error or a wrong theoretical calculation. In this particular case the previous experimental data on the proton charge radius do not seem to be rock-solid, at least to a casual observer. For example, if the charge radius is extracted from electron–proton scattering the discrepancy with the PSI measurement becomes only 3.1 sigma;
the PSI paper also quotes another recent measurement that is completely consistent with their result within error bars.

In any case, whenever a discrepancy with the standard model pops up, particle theorists cannot help thinking about new physics explanations. Our folk is notorious for ambulance chasing, but actually this is one of these cases when the ambulance is coming straight at us. Recently the particle community has invested a lot of interest in studies of light, hidden particles very weakly coupled to the ordinary matter. One example is the so-called dark photon: an MeV-GeV mass particle with milli-charge couplings to electrons and muons. This idea is pretty old, but in the past 2 years the interest in dark photons was boosted because their existence could explain certain astrophysical anomalies (Pamela). The signals of dark photons and other hidden particles are now being searched for at the Tevatron, LHC, B-factories, and in dedicated experiments such as ALPS at DESY, or APEX that is just kicking off at JLAB. No signal has been found in these experiments yet, but there is still a lot of room for the dark photon as long as its coupling to electrons and muons is $\epsilon \leq 10^{-3}$ smaller than that of the ordinary photon, see the picture borrowed from this paper. The news of the muonic Lamb shift came somewhat unexpectedly...but not to everyone: here is a passage from a 2-years old paper:

For example, the dark photon contribution to the electron-proton scattering amplitude at low momenta is equivalent to the $6 \epsilon^2 /m_A^2$ correction to the proton charge radius (...) It remains to be seen whether other precision QED tests (e.g. involving muonic atoms) would be able to improve on the current constraints.

So here we are. In the coming weeks we should see whether there exist concrete models capable of fitting all data. In any case, a new front in the battle against dark forces has just been opened. Now, could someone make us a muonium?

CDF says: calm down everybody

2010-05-26T20:28:00.004+01:00

Physics beyond the standard model has its ups and downs. Ups like mountains in the Netherlands, and downs like the Marianas Trench. Whenever something exciting seems to happen it's the telltale sign that a really big hammer is about to come down.

Last week the D0 experiment at the Tevatron presented the new measurement of the same-sign dimuon charge asymmetry in B-meson decays. This asymmetry probes CP violation in B-mesons, including the $B_s$ mesons that have been less precisely studied than their $B_d$ friends and may still hold surprises in store. D0 claimed that their measurement is inconsistent with the standard model at the 3.2 sigma level and hints to a new physics contribution to the $B_s \bar B_s$ mixing. 3 sigma anomalies in flavor physics are not unheard of, but in this case there were reasons to get excited. One was that the $B_s$ system is a natural place for new physics to show up, because the standard model contribution to the CP-violating mixing phase is tiny, and theoretical predictions are fairly clean. The other reason was that the D0 anomaly seemed to go along well with earlier measurements of CP violation in the $B_s$ system. Namely, the measurement of the $B_s$ decay to $J/\psi \phi$ displayed a 2.1 sigma discrepancy with the standard model, and some claimed the discrepancy is even higher when combined with all other flavor data. In other words, all measurements (except for $B_s \to D_s \mu X$ that however has a larger error) of the phase in the $B_s \bar B_s$ mixing consistently pointed toward new physics.

Not any more. Two days ago D0's rival experiment CDF presented crucial new results at the FPCP conference - a major sabbath of the flavor community. CDF repeated the measurement of the CP violation $B_s \to J/\psi \phi$ on a larger data sample of 5.2 inverse femtobarn, that is with 2 times larger statistics than in the previous measurement. And they see nothing: the result is 0.8 sigma consistent with the standard model.
So at this moment only one experiment claims to see an anomaly in the $B_s$ system, while another measurement of the $B_s \bar B_s$ mixing phase is perfectly consistent with the evil, corrupted standard model. The most likely hypothesis is that D0's result is a fluke and/or systematical uncertainties have been underestimated. Of course, further measurements of the mixing phase may bring another twist to the story...well i dont sound convincing, do I ;-)

Meanwhile at the LHC

2010-05-22T00:56:00.007+01:00

For the time being the most interesting physics results arrive from the Tevatron, as we were reminded this week by D0's announcement. The LHC cannot compete yet, but it's steadily working its way to becoming the leader sometime next year. According to the latest report, things are going pretty smoothly. So far the peak luminosity is $6x10^{28}/cm^2/s$ (corresponding to roughly an inverse picobarn per year), and the goal for the present run is to increase it by a factor of a thousand. Currently the machine people are working on increasing the numbers of protons in the bunches up to the nominal value of $\sim 10^{11}$. This step alone should allow them to reach $2x10^{29}/cm^2/s$ assuming just 2 bunches circulating in the LHC ring. After that, they will progressively add more and more bunches to the beam.
For the moment, the acquired luminosity is around 10 inverse nanobarns per experiment. This means that CMS and ATLAS have already collected almost 1000 W bosons (85 nanobarn cross section), hundreds of Z bosons (25 nanobarn cross section), and a few top quark pairs Poisson permitting (0.2 nanobarn cross section). ATLAS now shows on its public pages the first event displays with leptonically decaying Z bosons. The one reproduced above features a beautiful Z decaying into electrons (the two blobs in the electromagnetic calorimeter). Meanwhile, CMS has no new events on its public pages since the first collisions on March 30. The only logical explanation is that a giant octopus has eaten the detector together with the entire collaboration. As otherwise, if they had anything to share they would share it... or wouldn't they ;-)

New Physics Claim from D0!

2010-05-17T06:50:00.016+01:00

Tevatron not dead, or so it seems. Although these days all eyes are turned to the LHC, the old Tevatron is still capable to send the HEP community into an excited state. Last Friday the D0 collaboration presented results of a measurement suggesting the standard model is not a complete description of physics in colliders. The paper is out on arXiv now.

The measurement in question concerns CP violation in B-meson systems, that is quark-antiquark bound states containing one b quark. Neutral B-mesons can oscillate into its own antiparticles and the oscillation probability can violate CP (much as it happens with kaons, although the numbers and the observables are different). There are two classes of neutral B-mesons: $B_d$ and its antiparticle $\bar B_d$ where one bottom quark (antiquark) marries one down antiquark (quark), and $B_s,\bar B_s$ with the down quark replaced by the strange quark. Both these classes are routinely produced Tevatron's proton-antiproton collisions roughly in fifty-fifty proprtions, unlike in B-factories where mostly $B_d,\bar B_d$ have been produced. Thus, the Tevatron provides us with complementary information about CP violation in nature.

There are many final states where one can study B-mesons (far too many, that's why B-physics gives stomach contractions). The D0 collaboration focused on the final states with 2 muons of the same sign. This final state can arise in the following situation. A collision produces a $b \bar b$ quark pair which hadronizes to B and $\bar B$ mesons. Bottom quarks can decay via charged currents (with virtual W boson), and one possible decay channel is $b \to c \mu^- \bar \nu_\mu$. Thanks to this channel, the B meson sometimes (with roughly 10 percent probability) decays to a negatively charged muon, $B \to \mu^- X$, and analogously, the $\bar B$ meson can decay to a positively charged antimuon. However, due to $B \bar B$ oscillations B-mesons can also decay to a "wrong sign" muon: $B \to \mu^+ X$, $\bar B \to \mu^- X$. Thus oscillation allow the $B, \bar B$ pair to decay into two same sign muons a fraction of the times.

Now, in the presence of CP violation the $B \to \bar B$ and $\bar B \to B$ oscillation processes occur with different probabilities. Thus, even though at the Tevatron we start with the CP symmetric initial state, at the end of the day there can be slightly more -- than ++ dimuon final states. To study this effect, the D0 collaboration measured the asymmetry

$A_{sl}^b = \frac{N_b^{++} - N_b^{--}}{N_b^{++} + N_b^{--}}$.

The standard model predicts a very tiny value for this asymmetry, of order $10^{-4}$, which is below the sensitivity of the experiment. This is cool, because simply an observation of the asymmetry provides an evidence for contributions of new physics beyond the standard model.

The measurement is not as easy as it seems because there are pesky backgrounds that have to be carefully taken into account. The dominant background comes from ubiquitous kaons or pions that can sometimes be mistaken for muons. These particles may contribute to the asymmetry because the D0 detector itself violates CP (due to budget cuts the D0bar detector made of antimatter was never constructed). In particular, the kaon K+ happens to travel further than K- in the detector material and may fake a positive value of asymmetry. We have to cross our fingers that D0 got all these effects right and carefully subtracted them away. At the end of the day D0 quotes the measured asymmetry to be

$A_{sl}^b = -0.00957 \pm 0.00251(stat) \pm 0.00146 (syst)$,

that is the number of produced muons is larger than the number of produced antimuons with the statistical significance estimated to be 3.2 sigma. The asymmetry is some 100 times larger than the value predicted by the standard model!

Of course, it's too early to start dancing and celebrating the downfall of the standard model, as in the past the bastard have recovered from similar blows. Yet there are reasons to get excited. The most important one is that the latest D0 result goes well in hand with the anomaly in the $B_s$ system reported by the Tevatron 2 years ago. The asymmetry measured by D0 receives contributions from both $B_s$ and $B_d$ mesons. The $B_d$ mesons are much better studied because they were produced by tons in BaBar and Belle, and to everyone's disappointment they were shown to behave according to the standard model predictions. However BaBar and Belle didn't produce too many $B_s$ mesons (their beams were tuned to the Upsilon(4s) resonance which is a tad too light to decay into $B_s$ mesons), and so the $B_s$ sector can still hold surprises. Two years ago CDF and D0 measured CP violation in $B_s$ decays into $J/\psi \phi$, and they both saw a small, 2-sigma level discrepancy from the standard model. When these 2 results are combined with all other flavor physics data it was argued that the discrepancy becomes more than 3 sigma. The latest D0 results is another strong hint that something fishy is going on in the $B_s$ sector.

Both the old and the new anomaly prompts introducing to the fundamental lagrangian a new effective four-fermion operator that contributes to the amplitude of $B_s \bar B_s$ oscillations:

$L_{new physics} \sim \frac{c}{\Lambda^2}(\bar b s) ^2$ + h.c.,

with a complex coefficient $c$ and the scale in the denominator on the order of 100 TeV. At this point there are no hints from experiment what could be the source of this new operator, and the answer may even lie beyond the reach of the LHC. In any case, in the coming weeks theorists will derive this operator using extra dimensions, little Higgs, fat Higgs, unhiggs, supersymmetry, bricks, golf balls, and old tires. Yet the most important question is whether the asymmetry is real, and we're dying to hear from CDF and Belle. There will be more soon, I hope...

Official ICHEP what???

2010-05-13T18:06:00.004+01:00

Yes, what the say is true: ICHEP2010 has launched an official blog to cover the conference and signed up the cream of the blogosphere (including John Conway, Tommaso Dorigo, Micheal Schmitt). This is going to be an interesting experiment. ICHEP is a bi-annual series conferences with long tradition, probably the largest event in the field of high-energy physics. Blogging, on the other hand, is by many considered a subversive activity to which the most appropriate response is malleus maleficarum. ICHEP's initiative might be the first attempt on this scale to bring together these old and new channels of scientific communication. We'll see what happens...

So, I will be a part of it too (even if one might have expected they would pay me for *not* blogging about ICHEP, given my reputation ;-) July is going to be fun.

More dark entries

2010-05-01T21:59:00.018+01:00

I have another bucketful of dark matter news and gossips, some market fresh, some long overdue. Let me bullet it out, even if each may deserve a separate post.

The Xenon100 experiment in Gran Sasso - currently the most sensitive dark matter detection experiment on Earth - is up and running. The results from a short 11 days run in November last year were presented at the WONDER2010 conference a month ago. The signal region where nuclear recoils are supposed to appear is below the blue line. As you can see, bastards really have zero background events. Even this small amount of data allows them to set the limits on the dark matter - nucleon cross section comparable to those obtained by CDMS after many months of running. The experiment is continuously taking data since January and the plan is to run for an entire year. As of today they have roughly 10 times more data on tape, but it's not yet clear when the new chunk will be unblinded and analyzed. Can't wait.
Xenon100 can take their time because direct competitors are falling like flies. LUX, a US based experiment that relies on practically the same technology, is stranded until at least next year waiting for their underground cavern to be ready. WARP, a similar experiment next door in Gran Sasso but filled with argon rather than xenon as the target, was aborted last year due to an electrical failure. The latest (unconfirmed) rumor is that XMASS - a 1 ton xenon dark matter experiment in Japan - has been downed due to a simple engineering error. New York City psychics whisper in terror about dark ectoplasm currents sourced somewhere in northern Manhattan.
Back to Gran Sasso. CRESST's presentation at WONDER2010 devoted 1 slide to wild speculations about their latest unpublished results on dark matter detection. CRESST uses CaWO4 crystals as the target using and detect scintillation light and phonons to sort out the signal of dark matter recoiling on the nuclei making the crystal. The cool thing about the experiment is that using the light-to-phonon ratio they can to some extent tell whether a nuclear recoil occurred on tungsten or on oxygen. In the tungsten (blue) band, where weak scale dark matter is expected to show up first, there is almost no events. But in the oxygen band (reddish) there is something weird going on. Of course, most likely this is some sort of background that the collaboration has not pinned down yet. But another possible interpretation is that the dark matter particle is very light so that it bounces off heavy tungsten nuclei but still can give a kick to much lighter oxygen nuclei. Furthermore, the slide mentions that the event rate in the oxygen band displays a hint of annual modulation expected from dark matter scattering. Curiouser and curiouser...
...especially if CRESST data are viewed from a somewhat different angle. Juan Collar, apart from being a guest-blogger, has a daytime job at CoGeNT - another dark matter experiment that has recently seen hints of light dark matter particles. A few weeks ago during a workshop in New York Juan flashed the following plot (Content Warning: the plot below makes respectable physicists shout obscenities):
These are the CRESST data from the tungsten band plotted as the differential recoil spectrum. Naively, the spectrum fits the one expected from light dark matter particles of mass approximately 10 GeV, that is the same ballpark that also fits the CoGeNT data!
The situation could be clarified by the CDMS experiment. Although they finished data-taking, they are sitting on a large amount of data collected by their silicon detectors, of which only a part was analyzed and made public (their most recently published limits are based on data from the germanium detectors). Silicon is a fairly light element (A=28) and therefore it is more suitable than germanium for studying light dark matter. Thus CDMS has the potential to exclude the light dark matter interpretation of the CoGeNT and CRESST signals; unfortunately this does not seems to be their priority right now. CRESST itself should release a full-fledged analysis of their data soon, which should provide us with more solid information. However, CRESST at this point is not a background free experiment. Therefore in the nearest future we should expect a wilderness of mirrors rather than clear-cut answers. In other words, more rumors ahead :-)

Update: The paper with first Xenon100 results is now out on arXiv. The analysis chalenges the dark matter interpretation of the CoGeNT data. As you can see on the plot, the region of the parameter space favored by CoGeNT is excluded by Xenon100 at 90% confidence level. One should however note that these limits strongly depend on the quenching factor in xenon (that is how much of recoil energy gets converted into light). Different experimental measurements of that quenching factor point to different trends at low recoil energies (see fig.1 in the Xenon100 paper), which leaves some wiggle room.

Update #2: Just 2 days later Xenon100 gets a smackdown. A new paper by Collar and McKinsey casts doubt whether Xenon100 has any sensitivity to light dark matter particles consistent with the CoGeNT signal. As already hinted, Xenon100's assumptions about the quenching factor at low energies are controversial. Another assumption that is questioned concerns the distribution of the number of photoelectrons near threshold:

...limits depend critically (...) on the assumption of a Poisson tail in the modest number of photoelectrons that would be generated by a light-mass WIMP above detection threshold (...). We question the wisdom of this approach when the mechanisms behind the generation of any significant amount of scintillation are still unknown and may simply be absent at the few keVr level. To put it bluntly, this is the equivalent of expecting something out of nothing.

More Trouble with DAMA

2010-04-27T05:15:00.009+01:00

I haven't blogged about dark matter for almost 2 months, and already there is a pile of long overdue dark news. This post is about a couple of recent unpublished results that mean trouble for theorists trying interpret the DAMA signal.

Recall that the DAMA experiment has observed a few percent annual modulation of the recoil rate registered by their sodium-iodide crystal detector. This modulation could be due to a change of dark matter flux as the Earth moves around the Sun. However, other dark matter detection experiments (maybe except for CoGeNT) do not observe any signal, which puts strong constraint on the properties of the dark matter particle that could explain all available data. Vanilla-flavor models are by far excluded, however until recently two slightly more involved yet still plausible scenarios appeared marginally allowed:

Weak scale inelastic dark matter. In this scenario a dark matter particle with mass of order 100 GeV scatters to an excited state with order 100 keV mass splitting. The inelastic scenario favors heavy targets (such as DAMA's iodine, A = 127), and enhances the modulation rate (only dark matter particles from the tail of the velocity distribution can scatter, so that small changes of Earth velocity can significantly change the available phase space).
Light (5-10 GeV) elastic dark matter. This scenario favors very light targets (such as DAMA's sodium, A = 23) and experiments with low detection thresholds (such as DAMA's 2 keV), as light dark matter particles cannot give a large push to heavier target nuclei.

A few weeks ago, the former possibility was blasted by CRESST - yet another dark matter experiment under Gran Sasso mountain. CRESST uses CaWO4 crystals as the target, and detects scintillation and phonons to discriminate nuclear recoils (expected from dark matter particle) from alpha, beta, and gamma recoils (induced by ubiquitous backgrounds). The presence of tungsten (A = 184) in their crystal makes it very sensitive to the inelastic scenario. But the latest preliminary results presented at the Wonder2010 conference do not show a clear signal. Although CRESST has a handful of (most likely) background events in the signal band, the number of hits is much smaller than that predicted by the inelastic scenario consistent with the DAMA signal. The collaboration claims that the DAMA region is excluded by more than 3 sigma. This should be treated with a grain of sodium chloride as the CRESST data are not yet public and the assumptions that enter the derivation of the limits are not clearly spelled out in the slides. But most likely, the inelastic window is getting closed.

The explanation of DAMA via a 5-10 GeV dark matter particle is also facing problems. The (marginal) consistency of this scenario with null results from other experiments hinges on the so-called channeling effect in sodium-iodide crystals. Normally, an incoming particle recoiling against the crystal nuclei deposits most of the recoil energy in the form of lattice excitations (not observed by DAMA) while only a small fraction goes into scintillation (observed by DAMA). Channeling refers to the situation when an incoming particle gets caught along the symmetry plane of the crystal undergoing a series of small-angle scatterings and losing most of its energy via scintillation. Since a fraction of less energetic recoils can be detected thanks to channeling, the detection threshold of the experiment is effectively lowered. The effect is especially important for light dark matter because in this case the recoil spectrum is very sharply peaked toward lower energies. The channeling probability reported by DAMA is very large, of order 30 percent in the interesting range of recoil energies, which would greatly increase their sensitivity to light dark matter.

Given its importance you may expect that channeling in sodium-iodide crystals has been carefully studied by the DAMA collaboration. However, DAMA would not be herself if she dwelled on such trivialities. Instead the collaboration estimated the channeling probability using monte carlo simulations based on a theoretical model not applicable for the actual problem. Recently I came across slides from the Snowpac2010 workshop describing an independent attempt to estimate the channeling fraction using more reliable theoretical assumptions. The preliminary results contradict the conclusion of the DAMA collaboration: the channeling probability in sodium-iodine is negligible. If this is right, simple models of light dark matter cannot consistently explain the DAMA oscillation results.

Assuming that both of these preliminary results are true, we are confronted with an embarrassing situation: there is no plausible theoretical interpretation of the DAMA results. What remains on the market are rather exotic models (e.g. resonant dark matter) or Frankenstein models that patch up several non-trivial effects (inelastic+form factor, inelastic+streams, and so on). So theorists need to think harder. At the same time, the need to independently verify the DAMA experimental results becomes even more acute. Maybe a socially sensitive hacker could upload DAMA's raw data on WikiLeaks ;-)

Another Anomaly from CDF

2010-04-12T06:41:00.012+01:00

The CDF multimuon anomaly that hit the news 18 months ago is now almost forgotten (though not quite explained away). It appears that Tevatron's CDF results harbor yet another disturbing anomaly. The story goes back to an innocuous measurement of the transverse momentum of charged particles in minimum bias events. In particle physics slang, minimum bias stands for boring, routine measurements. Minimum bias events typically feature soft (low momentum transfer) QCD interactions between colliding hadrons, and normally would not be even recorded on tape because they happen too oftent and do not contain anything we consider interesting (like hard jets, electrons, muons, or missing energy). Only a small random subset of minimum bias events is kept to provide a control sample for tuning monte carlo simulations of hadronic collisions.

The main result of that CDF study is plotted on the right. The dashed red line is a prediction of monte carlo simulations, while the solid green line is just a line drawn over the data points to make us feel secure. As you can see, the data are well described by simulations up to transverse momentum of order 20 GeV. However, around 100 GeV there is a huge, some 3 orders of magnitude discrepancy! The study concludes

...A comparison with a pythia prediction at the hadron
level is performed. The inclusive charged particle differential production cross section is fairly well reproduced only in the transverse momentum range available from previous measurements. At higher momentum the agreement is poor.The dependence of the charged particle transverse momentum on the particle multiplicity needs the introduction of more sophisticated particle production mechanisms, such as multiple parton interactions, in order to be better explained...

that is to say, it's strange but who cares...

Fast forward. A year later a number of theorists began to ponder on the CDF result. Last month, Albino et al. concluded that the discrepancy is too large to be swept under the carpet of theoretical errors, and went as far as suggesting a violation of the QCD factorization theorem. Somewhat later, however, Cacciari et al argued that even such a radical proposal is not a viable explanation. They observed that for pT of order 100 GeV the charged-particle cross section measured by CDF becomes comparable to the jet cross section measured elsewhere. This means that the excess cannot have anything to do with QCD-like events (unless one assumes that jets in this momentum regime contain on average one high-pT charged particle, which is both absurd and inconsistent with measured particle distributions within jets). If the effect is real, the culprit events must be very different from QCD so that they do not affect the measured jet distributions.

Could this be new physics then? High-pT tracks could be left by heavy long lived particles (the likes of charginos or staus in some versions of gauge mediation, or R-hadrons in split supersymmetry).The problem, much as in the case of the CDF multimuon anomaly, is the huge cross section of order tens of nanobarns required to fit the data. Recall that typical models of new physics at the weak scale predict cross sections at least ten thousand times smaller - of order picobarns or less. For example, a 10 nanobarn cross section would correspond to a 20 GeV gluino. It is hard to understand how new physics produced in such large quantities could have escaped detection by multiple searches at the Tevatron. So far, no one has come up with an even remotely viable new physics model explaining the high-pT anomaly from CDF.

At this stage, the most likely explanation is that the anomaly is an experimental error. Maybe a small subset of tracks was misreconstructed so that they appear to have larger momenta than they really have, or maybe a grad student accidentally spilled coffee on the data. Nevertheless, it is mind-boggling that such a large chunk of intriguing data could pass completely unnoticed for so long, just because the discrepancy showed up in a different place than everybody was looking. What else is hiding in 7 inverse femtobarn of data acquired so far by the Tevatron experiments?

Meanwhile at the Tevatron: ...only wind is blowing through deserted corridors full of rubble, broken glass and bird droppings. The humans who used to work here have vanished inside the CERN black hole, only few survivors cower in the Higgs search office. The accelerator is running by sheer inertia spitting out rolls of paper filled with data which pile up in the basements where rats feed on them...

W bosons back at CERN

2010-04-07T15:49:00.009+01:00

No words needed, the pictures say it all. Welcome back, after 10 almost years! Apparently, the LHC has already acquired nearly 1 inverse nanobarn of luminosity, one milionth of what is planned for the 2010-2011 run. By now there must be several hadronically decaying W bosons on tape (W decays to an electron or muon only 10 percent of the times each). Now we're dying to see first LHC Z bosons, and soon enough first European top quarks :-)

More ATLAS events here.

Farewell to the Noughties - Theory

2010-04-03T05:19:00.006+01:00

The LHC just started colliding protons at 7 TeV, marking the symbolic beginning of a new decade in particle physics. A good moment to complete the summary of the past one. Some time ago I made a list of most important particle-related experimental results. Time for theory. What significant developments took place in particle theory during the noughties?

Well...none, in the first approximation. The past decade has been marked by inertia and intellectual masturbation. The last truly novel ideas, like AdS/CFT, Randall-Sundrum, or ADD, were all born back in the 90s. No surprise that the list of 50 top cited articles last year contains only 3 particle theory papers written in the 00s, none of which in a prominent position.

Nevertheless, our understanding of particle theory has progressed, somewhat. Here is my subjective, biased, and utterly unfair summary of the most interesting developments.

Extra dimensions are strong dynamics
Warped extra dimensions a-la Randall-Sundrum have dominated new physics model building. Perhaps the most interesting aspect of that industry is a qualitative analogy between five-dimensional warped models and purely four-dimensional strongly coupled models. This of course is not completely unexpected given the AdS/CFT conjecture. Nonetheless it is interesting that the correspondence extends to more down-to-Earth and phenomenologically relevant examples, even if in a vulgarized form. And so, 5D Higgsless theories are large N technicolor models in disguise, 5D gravity in a black hole background captures some aspects of heavy ion physics, etc. Even low-energy QCD can, to a certain extent, be modeled this way, and some quantitative predictions for the parameters of the effective chiral lagrangian can be derived.
Higgs can be stabilized without supersymmetry
The bulk of particle theory is driven by the fact that the Higgs boson mass in the standard model receives large, quadratically divergent corrections at the quantum level. The common expectation, or maybe just wishful thinking, is that new symmetries and new particles appear at the TeV scale to fix that problem. The best known example - supersymmetry - is based on a boson-fermion interplay, for example, quantum corrections from the top quark are canceled by its scalar partners called stops. This is however not the only possibility, and the cancellations can occur between the same-statistics particles, for example top quark contributions can be canceled by another heavy colored fermion. This option has been known since 70s, but only during the last decade it was systematically understood and classified in the framework of little Higgs and gauge-Higgs unification theories. The end results is rather depressing though: all models we have constructed so far are just as good, or rather just as bad, as supersymmetry.
QCD is boring but it's here to stay
Theorists working on QCD have always been looked down upon by smartasses building new fancy models of the universe. Yet in the past and in the coming decade hadron colliders are the sad reality, and an input from QCD theory is necessary to isolate new physics from mundane background processes. Definitely, tons of good work in that direction has been done. At the most basic level, we have seen heroic computations of higher-order corrections to SM processes like W+jets, Z+jets or ttbar+jets, and so on, without which life at the LHC would be much harder. At a more sophisticated level, new jet algorithms better suited for hadron colliders have been developed, and new ways to search for new physics using jet substructure have been proposed. One should also mention the progress in theoretical handling of QCD, for example the soft-collinear effective theory.
There is more to dark matter than meets the eye
Models of dark matter are more numerous than stars in the sky, so why bother about another thousand spawned during the last decade? However, some recent proposals are important because they changed the way we search for dark matter. On one hand, models based on KK parity and T-parity prompted us to explore new collider signatures. Even more important was the impact on direct detection experiments. Not so long ago experimenters, brainwashed by MSSM preachers, searched only for spin-independent (coupled to nucleus' mass) or spin-dependent (coupled to nucleus' spin) elastic WIMP scattering. Experimental set-ups as well as data analyses were tailored for these 2 possibilities to the point that less standard dark matter signals would simply be discarded as background. This embarassing situation has been greatly improving in recent years. Thanks in part to inelastic dark matter models, or the recent offensive of light GeV scale elastic dark matter models, experimental analyses are becoming more flexible and developing alternative experimental techniques is being encouraged.
There are more ways to compute scattering amplitudes
Anybody who ever computed scattering amplitudes in gauge theories can't help the feeling that there is something wrong with the standard way of doing it. In the approach via Feynman diagrams, hundreds of complicated expressions at the end of the day magically combine into something far more simple. It is becoming more and more clear that gauge theories may hide surprising mathematical structures that control scattering amplitudes. During the last decade some of these structures have been uncovered thanks to e.g. BCFW recursion relations, CSW rules, or fancy twistor space techniques. More recently, a new approach based on Grassmannians suggests that the hidden simplicity extends to higher loop levels, at least in the maximally supersymmetric case. But this last one might be more appropriate for my Farewell to the Teenies...

So much for the last decade, now dying to see the new one. Clearly, it can't get much worse :-)