Eur. Phys. J. Plus (2015) 130: 184
https://doi.org/10.1140/epjp/i2015-15184-3

Regular Article

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

¹ Sezione di Pisa, INFN, Pisa, Italy
² Università di Pisa, Pisa, Italy

^* e-mail: luca.martini@cern.ch

Received: 31 May 2015
Revised: 15 July 2015
Accepted: 5 August 2015
Published online: 14 September 2015

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.