Abstract

A search for the rare decays $B^0_s \to \mu^+\mu^-$ and $B^0 \to \mu^+\mu^-$ 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 $B^0_s \to \mu^+ \mu^-$ candidates with respect to the background expectation. The probability that the background could produce such an excess or larger is 5.3 x 10$^{-4}$ corresponding to a signal significance of 3.5 standard deviations. A maximum-likelihood fit gives a branching fraction of $BR(B^0_s \to \mu^+ \mu^-) = (3.2^{+1.5}_{-1.2})$ x $10^{-9}$, where the statistical uncertainty is 95% of the total uncertainty. This result is in agreement with the Standard Model expectation. The observed number of $B^0 \to \mu^+ \mu^-$ candidates is consistent with the background expectation, giving an upper limit of $BR(B^0 \to \mu^+ mu^-) < 9.4$ x $10^{-10}$ at 95% confidence level.