A search for the rare decays $B^0_s\rightarrow \mu^+\mu^-$ and $B^0\rightarrow \mu^+\mu^-$ is performed with the LHCb experiment using 1.1\,fb$^{ -1}$ of data collected at $\sqrt s=8$\,TeV$ and 1.0\,fb$^{ -1}$ of data collected at $\sqrt s=7$\,TeV. An excess of $B^0_s\rightarrow \mu^+\mu^-$ candidates with respect to the background expectations is observed with a statistical significance of 3.5 standard deviations. A branching fraction of ${ \cal BR}(B^0_s\rightarrow \mu^+\mu^-) =(3.2^{ +1.5}_{ -1.2}) \times 10^{ -9}$ is measured with an unbinned maximum likelihood fit. The measured branching fraction is in agreement with the expectation from the Standard Model. The observed number of $B^0\rightarrow \mu^+\mu^-$ candidates is consistent with the background expectation and an upper limit on the branching fraction of ${ \cal BR}(B^0\rightarrow \mu^+\mu^-) < 9.4\times10^{ -10}$ is obtained.
Categories: hep-ex hep-ph