What if they don't find the Higgs?

(...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

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

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

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

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

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

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 :-)