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

Abstract

We describe the CMS measurement of the branching fraction of the decay B ⁰ _s → 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 ⁰ _s 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 ⁰ _s → μ ⁺ μ ⁻ branching fraction is B(B ⁰ _s → μ ⁺ μ ⁻) = (3.0 ^+1.0_−0.9 × 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.