Sunday, 21 December 2014

Weekend plot: rare decays of B mesons, once again

This weekend's plot shows the measurement of the branching fractions for neutral B and Bs mesons decays into  muon pairs:
This is not exactly a new thing. LHCb and CMS separately announced evidence for the B0s→μ+μ- decay in summer 2013, and a preliminary combination of their results appeared a few days after. The plot above comes from the recent paper where a more careful combination is performed, though the results change only slightly.

A neutral B meson is a  bound state of an anti-b-quark and a d-quark (B0) or an s-quark (B0s), while for an anti-B meson the quark and the antiquark are interchanged. Their decays to μ+μ- are interesting because they are very suppressed in the standard model. At the parton level, the quark-antiquark pair annihilates into a μ+μ- pair. As for all flavor changing neutral current processes, the lowest order diagrams mediating these decays occur at the 1-loop level. On top of that, there is the helicity suppression by the small muon mass, and the CKM suppression by the small Vts (B0s) or Vtd (B0) matrix elements. Beyond the standard model one or more of these suppression factors may be absent and the contribution could in principle exceed that of the standard model even if the new particles are as heavy as ~100 TeV. We already know this is not the case for the B0s→μ+μ- decay. The measured branching fraction (2.8 ± 0.7)x10^-9  agrees within 1 sigma with the standard model prediction (3.66±0.23)x10^-9. Further reducing the experimental error will be very interesting in view of observed anomalies in some other b-to-s-quark transitions. On the other hand, the room for new physics to show up  is limited,  as the theoretical error may soon become a showstopper.

Situation is a bit different for the B0→μ+μ- decay, where there is still relatively more room for new physics. This process has been less in the spotlight. This is partly due to a theoretical prejudice: in most popular new physics models it is impossible to generate a large effect in this decay without generating a corresponding excess in B0s→μ+μ-. Moreover,  B0→μ+μ- is experimentally more difficult:  the branching fraction predicted by the standard model is (1.06±0.09)x10^-10, which is 30 times smaller than that for  B0s→μ+μ-. In fact, a 3σ evidence for the B0→μ+μ- decay appears only after combining LHCb and CMS data. More interestingly, the measured branching fraction, (3.9±1.4)x10^-10, is some 2 sigma above the standard model value. Of course, this is  most likely a statistical fluke, but nevertheless it will be interesting to see an update once the 13-TeV LHC run collects enough data.

U said...

The excess in the B0 search might also be simply a systematic effect, as the background situation is quite different for the B0 compared to the Bs.

andrew said...

While it seems to be fair to say that the CP violation data from kaons, neutral B mesons and neutral D mesons, is consistent with the SM explanation of a single CKM matrix CP violating phase; the MOE on that is quite large (the two SD range is like 60 degrees wide), and IIRC, some of the measured CP violation in D mesons in even of the wrong sign and takes some contortions to reach the right theoretical estimate which is like 50-100 times as great as the naive estimate with too many simplifying assumptions in the calculation.

I'm a bit surprised that this isn't a more fruitful area for BSM physics articles (not that there aren't some proposals out there).

mfb said...

50-100 times? There were some delta A_cp measurements larger than expected (~factor of 5), but the central value went down again with more data, and theory prediction went up with more work done (and the uncertainty estimate got better). The value is difficult to predict, but theory and experiment look consistent again now.