Bs to mumu Observed

The LHCb experiment has observed the rare decay B_s→μ⁺μ^–.

Br(B_s→μ⁺μ^–) = (3.2^+1.5_-1.2)×10^-9.

This is the culmination of nearly 30 years of searching for this extremely rare decay (see yesterday’s blog post).

The slides from Johannes Albrecht presented at the HCP conference give a nice overview of the measurement. These results come from 1 fb^-1 at 7 TeV and 1.1 fb^-1 at 8 TeV, analyzed together with methods very similar to those published in March. For this update, evaluations of the background shapes have been refined and are better constrained by the data, leading to a reduction in the systematic uncertainty. There is also an improvement in f_s, the parameter that gives the fraction of b-quarks the hadronize to form a B_s meson.

Here is the limit curve, showing clearly that the data are incompatible with the brackground hypothesis:

Limit curve

In fact, the p-value for the 2011+2012 data is 5×10^-4 corresponding to 3.5σ. Conventionally speaking, this is certainly good enough for evidence.

This is the ideal case for a mass peak, since the mass resolution is excellent (25 MeV) and the background is almost zero. Here is a plot showing a B_s signal peak emerging from the background as the cut on the BDT (boosted decision tree, a common multivariate analysis tool) is increased:

The Bs peak emerges as the cut on the BDT is increased.

The measured rate, Br(B_s→μ⁺μ^–) = (3.2^+1.5_-1.2)×10^-9 is compatible with the SM prediction, (3.2±0.2)×10^-9, leaving very little room for new physics contributions. It will be interesting to see how this measurement constrains models of new physics.

Hopefully CMS will also report observation of this rare decay mode, confirming the LHCb result.