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 ;-)
Saturday, 27 August 2011
Tuesday, 23 August 2011
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?
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.