First Evidence for the Decay B[subscript s][superscript 0]→μ[superscript +]μ[superscript -]

A search for the rare decays B[subscript s][superscript 0]→μ[superscript +]μ[superscript -] and B[superscript 0]→μ[superscript +]μ[superscript -] 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[superscript -1] of proton-proton collisions at √s=8 TeV and 1.0 fb[superscript -1] at √s=7 TeV. We observe an excess of B[subscript s][superscript 0]→μ[superscript +]μ[superscript -] candidates with respect to the background expectation. The probability that the background could produce such an excess or larger is 5.3×10[superscript -4] corresponding to a signal significance of 3.5 standard deviations. A maximum-likelihood fit gives a branching fraction of B(B[subscript s][superscript 0]→μ[superscript +]μ[superscript -])=(3.2[subscript -1.2][superscript +1.5])×10[superscript -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[superscript 0]→μ[superscript +]μ[superscript -] candidates is consistent with the background expectation, giving an upper limit of B(B[superscript 0]→μ[superscript +]μ[superscript -])<9.4×10[superscript -10] at 95% confidence level.