The CMS observation of the rare decay B s 0 → μ + μ − with the LHC Run I data

Abstract

We describe the CMS measurement of the branching fraction of the decay B s 0 → μ + μ −, and the upper limit of the decay B 0 → μ + μ −, performed with the full data of LHC Run I. For the B s 0 decay, results are extracted from an unbinned maximum likelihood fit to the dimuon invariant mass distribution in twelve independent categories. The excess of signal events with respect to background corresponds to a significance of 4.3 standard deviations, and the measured B s 0 → μ + μ − branching fraction is B(B s 0 → μ + μ −) = (3.0 −0.9 +1.0 × 10−9). For the decay B 0 → μ + μ −, a counting experiment with the CLs method is performed in the B 0 mass window and an upper limit of B(B 0 → μ + μ −) < 1.1 × 10−9 at the 95% confidence level is estimated. Both results are in agreement with the expectations of the standard model and with the recent measurements of the LHCb Collaboration.