### Search for the rare decays $B \rightarrow \mu^+\mu^-$ with the CMS detector.

Franco Simonetto
I summarize here the results of a search for the rare decays $B^0 \rightarrow \mu^+\mu^-$ and $B^0_s\rightarrow \mu^+\mu^-$, based on a sample of data collected by the CMS detector from $pp$ collisions at $\sqrt{s} =7$ \TeV, corresponding to an integrated luminosity of 4.9 \invfb . No excess of events over the expected background is observed. The resulting upper limits on the branching fractions are \mbox{${\cal B}(B_s^0 \rightarrow \mu^+ \mu^- ) < 7.7 \times 10^{-9}$} and...