The CMS observation of the rare decay B$_{s}^{0}$ $\rightarrow \mu^{+} \mu^{-}$ with the LHC Run I data

We describe the CMS measurement of the branching fraction of the decay B$_{s}^{0}$ $\rightarrow \mu^{+} \mu^{-}$, 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 $\mathcal{B}$($B_{s}^{0}$ → 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 $\mathcal{B}$($B^{0}$ → 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.