Wednesday 26 May 2010

CDF says: calm down everybody

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 ;-)

Saturday 22 May 2010

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 ;-)

Monday 17 May 2010

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...

Thursday 13 May 2010

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.

Saturday 1 May 2010

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.