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 ;-)