Measurement of the Bs0→μ+μ- Branching Fraction and Search for B0→μ+μ- with the CMS Experiment

Results are presented from a search for the rare decays B_s⁰→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 20fb^-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(B_s⁰→μ⁺μ^-)=(3.0_-0.9^+1.0)×10^-9, where the uncertainty includes both statistical and systematic contributions. An excess of B_s⁰→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^-)<1.1×10^-9 at the 95% confidence level is determined. Both results are in agreement with the expectations from the standard model.