Measurement of the B⁰_s → μ⁺μ^- Branching Fraction and Search for B⁰ → μ⁺μ^- with the CMS Experiment

Results are presented from a search for the rare decays B⁰_s → μ⁺μ^- and B⁰ → μ⁺μ^- in pp collisions at √s = 7 and 8 TeV, with data samples corresponding to integrated luminosities of 5 and 20 fb^-1, respectively, collected by the CMS experiment at the LHC. An unbinned maximum-likelihood fit to the dimuon invariant mass distribution gives a branching fraction B(B⁰_s → μ⁺μ^-) = (3.0^+1.0_-0.9) x 10^-9, where the uncertainty includes both statistical and systematic contributions. An excess of B⁰_s → μ⁺μ^- events with respect to background is observed with a significance of 4.3 standard deviations. For the decay B⁰ → μ⁺μ^- an upper limit of B(B⁰ → μ⁺μ^- ) < 1.1 x 10^-9 at the 95% confidence level is determined. Both results are in agreement with the expectations from the standard model.