Here we are, just eight days away from the start of the EPS conference, the first big meeting of the summer conference season. You can bet that all of the LHC experiments are racing to come up with results that will make as big a splash as possible there. (Or, failing that, at DPF two and a half weeks later, or Lepton-Photon two weeks after that.)
However, even before then, we have the first interesting result of the season in a paper submitted by CDF just yesterday. The topic is a search for a Bs meson that decays to a μ+μ– pair. The Bs is made of a bottom antiquark and strange quark, and a student in an introductory particle physics course would (or should) tell you immediately that this decay can’t happen, as the standard model doesn’t allow for any “flavor changing neutral currents”, and there is no means for these two quarks of different flavors to annihilate each other. But a student in the next course would (or should) point out that no FCNC only applies to leading-order processes, and that there are higher-order, more rare interactions that would allow or this. Still, the rate is expected to be extremely low, with a branching ratio of about 3.2 x 10-9. Various models of new physics, including supersymmetry, predict that there should be additional interactions that would lead to a higher rate for this decay. So far, the decay has never been observed; the best measured upper limit is 4.2 x 10-8, more than an order of magnitude above the SM prediction.
The new CDF result, however, provides a tantalizing hint of a signal. Their latest analysis uses more data than their previous (2008) analysis, is sensitive to 20% more of the signal decays, and has improved the signal to background by a factor of two, to give a factor of 3.3 increased sensitivity overall. There is an excess of signal events over the predicted background, and if interpreted as a Bs signal gives a branching fraction of (1.8 +- 1.0) x 10-8. This is about two standard deviations away from zero — not significant enough to be called a discovery, by any means (CDF predicts that there is about a 2% chance that this is a background fluctuation), but interesting. If it does hold up, then the branching ratio would be about five times bigger than that expected in the SM. (Insert your own speculations here.)
So the question we must ask here is: what does the LHC have to say about this? The b-hadron production rate of the LHC is huge, perhaps an order of magnitude above that of the Tevatron, and so the experiments can have similar sensitivity with less integrated luminosity. This measurement is one of the goals of the LHCb experiment; they have a result based on the 2010 data sample alone setting an upper limit of 5.6 x 10-8, which is not in conflict with the CDF result. With the LHC having delivered about 30 times as much data in 2011 compared to 2010, LHCb will surely have increased sensitivity. ATLAS and CMS also have active b-physics programs, and will be able to look for this decay. Of course we are all very eager to find out if these experiments will see results consistent with CDF or not. If they do — and this point we can merely speculate — it’s going to be very interesting news. If not, well, sometimes life gives you statistical fluctuations.
Coming soon? We can hope.
