First Evidence for the Decay B0s→μ+μ−

A search for the rare decays B 0 s → μ + μ − and B 0 → 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 B 0 s → 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 ( B 0 s → μ + μ − ) = ( 3.2 + 1.5 − 1.2 ) × 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 B 0 → μ + μ − candidates is consistent with the background expectation, giving an upper limit of B ( B 0 → μ + μ − ) < 9.4 × 10 − 10 at 95% confidence level