First Evidence for the Decay B0s→μ+μ−

Abstract

A search for the rare decays B0s→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 B0s→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(B0s→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 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.