Saturday 27 August 2011

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 ;-)

12 comments:

Anonymous said...

LHCb is yet to rule out the second region. Did they show anything for the dimuon? They were supposed to come up with a sum-type plot vis-a-vis the difference plot for D0.

Jester said...

No, the a_sl^s - a_sl^d measurement is not ready yet. The other new thing they showed was B0->K*mu+mu- as a function of the mu+mu- invariant mass, and, surprise, it agrees with the SM.

Anonymous said...

A good plot for conspiracy theorists. Just having ideas for my next (and first) book ...

1. Nature showed her new face to the American experiments. (Well, she must, because the US is the chosen country, even if AA+.) But the Europeans conspired to knock off all American results. (I have to explain the American collaborations at the LHC though. Any idea?)

2. Nature is taking her revenge on the Europeans by hiding the Higgs. Ultimately, CDF will find it on its last day!

:-)

Kea said...

Nice, but as anon points out, we were hanging out for the new dimuon result.

Anonymous said...

Forget dimuons, all these US anomalies are wrong. Just apply the correct conversion rate:

CHsigma = 1.5 USsigma

3 sigma in the US = 2 sigma in Europe

Anonymous said...

Why should we assume a priori that the US experiments were wrong? We should take both data sets and average. For example, the beta_s average is still almost 2 sigma away from SM. After all Tevatron experiments have an order of magnitude more data than LHCb! Ultimately, of course, Tevatron will get swamped by LHC, but who knows, LHC may start to get those anomalous signals when it's time to shut down!

Anonymous Snowboarder said...

Just curious why everyone is assuming that D0 is wrong and LHCb is right? D0 and CDF have been running for ages and are well understood detectors. Can the same be said with the same confidence of LHCb (or any of the LHC detectors?)

Jester said...

D0's dimuon charge asymmetry is a freaking difficult measurement, with a number of hard-to-control detector effects coming on top of the eventual signal. Note that CDF was not able to repeat it so far. In comparison, the measurement of the Bs mixing phase in decays to J/Psi Phi seems cleaner. That's why I think it's more probable that D0 rather than LHCb got it wrong. But of course the matter is not settled: it may be the other way around, or both results may be correct in some perverse new physics scenarios.

Anonymous said...

Jester, you've said a bunch of times that the LHC *has* to find new physics, because the SM predictions break down around one TEV. Googling, I think what you meant is that unitarity seems to be violated in terms of the proportion of Ws and Zs produced, but I'm one of those people who doesn't really understand this stuff until we get it in simple English, so maybe I have that totally wrong.

Anyway, can you explain what results the LHC has found in that sector where you believe the SM predictions break down?

Kea said...

It is not perverse to suggest that triality, a cornerstone of M theory, is responsible for both results being correct.

Thomas Dent said...

It is a law of nature that B physics must yield approximately one 3 sigma deviation from the SM every 1 or 2 years, which persists just long enough for approximately 10 theorists to write papers showing that it confirms their favourite model, then disappears by the time of the next big conference.

Since there are so many processes to measure over such a wide range of branching fractions, there will always be something uncertain enough to speculate about. It's a fertile field, but remember what you must spread on fields to keep them fertile ...

Andrew Oh-Willeke said...

I was quite surprised by this one. There were lots of well motivated reasons for B mesons to show beyond the standard model physics and more than one distinct B meson related result had been anomalous.