First evidence of the B0s -> mu+ mu- decay

A search for the rare decays Bs -> mu+ mu- and B0 -> mu+ mu- is performed using 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 sqrt(s)=8 TeV and 1.0 fb-1 at sqrt(s)=7 TeV. We observe an excess of Bs -> mu mu candidates with respect to the background expectation. The probability that the background could produce such an excess or larger is 5.3x10-4 corresponding to a signal significance of 3.5 standard deviations. A maximum-likelihood fit gives a branching fraction of B(Bs -> mu+ mu-)=(3.2+1.5-1.2)x10-9, 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 -> mu+ mu- candidates is consistent with the background expectation, giving an upper limit of B(B0 -> mu+ mu-)<9.4x10-10 at 95% confidence level.