Measurement of the Bs0→μ⁺μ^- branching fraction and search for B0→μ⁺μ^- decays at the LHCb experiment

A search for the rare decays Bs0→μ ⁺μ ^- and B0→μ ⁺μ ^- is performed at the LHCb experiment. The data analyzed correspond to an integrated luminosity of 1 fb ^-1 of pp collisions at a center-of-mass energy of 7 TeV and 2 fb ^-1 at 8 TeV. An excess of Bs0→μ ⁺μ ^- signal candidates with respect to the background expectation is seen with a significance of 4.0 standard deviations. A time-integrated branching fraction of B(Bs0→μ ⁺μ ^-)=(2.9-1.0+1.1)×10 ^-9 is obtained and an upper limit of B(B0→μ ⁺μ ^-)<7.4×10 ^-10 at 95% confidence level is set. These results are consistent with the standard model expectations.