A search for the rare decays $B^0_s \to\mu^+\mu^-$ and $B^0 \to\mu^+\mu^-$ is performed at the LHCb experiment. The data analysed correspond to an integrated luminosity of 1 fb$^{ -1}$ of $pp$ collisions at a centre-of-mass energy of 7 TeV and 2 fb$^{ -1}$ at 8 TeV. An excess of $B^0_s \to\mu^+\mu^-$ signal candidates with respect to the background expectation is seen with a significance of 4.0 standard deviations. A time-integrated branching fraction of ${ \cal B}(B^0_s \to\mu^+\mu^-) = (2.9^{ +1.1}_{ -1.0})\times 10^{ -9}$ is obtained and an upper limit of ${ \cal B}(B^0 \to\mu^+\mu^-) < 7.4\times 10^{ -10}$ at 95% confidence level is set. These results are consistent with the Standard Model expectations.
Categories: hep-ex hep-ph