Meanwhile at the LHC

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!

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???

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

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

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:

1. 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).
2. 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

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

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.