First Evidence for the Decay B_{s}^{0}→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.