First evidence for the decay Bs0→μ+^μ-

A search for the rare decays Bs0→μ ⁺μ ^-and B0→μ ⁺μ ^-is performed with data collected in 2011 and 2012 with the LHCb experiment at the Large Hadron Collider. The data samples comprise 1.1 fb ^-1 of proton-proton collisions at √s=8 TeV and 1.0 fb ^-1 at √s=7 TeV. We observe an excess of Bs0→μ ⁺μ ^-candidates with respect to the background expectation. The probability that the background could produce such an excess or larger is 5.3×10 ^-4 corresponding to a signal significance of 3.5 standard deviations. A maximum-likelihood fit gives a branching fraction of B(Bs0→μ ⁺μ ^-)=(3.2-1.2+1.5) ×10 ^-9, where the statistical uncertainty is 95% of the total uncertainty. This result is in agreement with the standard model expectation. The observed number of B0→μ ⁺μ ^-candidates is consistent with the background expectation, giving an upper limit of B(B0→μ ⁺μ ^-)<9.4×10 ^-10 at 95% confidence level.