LHCb: First evidence for the decay $B^0_s \rightarrow \mu^+\mu^-$

A search for the rare decays $B_s \rightarrow \mu^+ \mu^-$ and $B_d \rightarrow \mu^+ \mu^-$ 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 $\sqrt{s} = 8$ TeV and 1.0 fb$^{-1}$ at $\sqrt{s}=7$ TeV. We observe an excess of $B_s \rightarrow \mu^+ \mu^-$ candidates with respect to the background expectation. The probability that the background could produce such an excess or larger is $5.3\times 10^{-4}$ corresponding to a signal significance of 3.5 standard deviations. A maximum-likelihood fit gives a branching fraction of $ {\cal B} (B_s \rightarrow \mu^+ \mu^-)$ = $(3.2^{\,+1.5}_{\,-1.2}) \times 10^{-9}$, where the statistical uncertainty is 95 % of the total uncertainty. This result is in agreement with the Standard Model expectation. The observed number of $B^0 \rightarrow \mu^+ \mu^-$ candidates is consistent with the background expectation, giving an upper limit of ${\cal B}(B_d \rightarrow \mu^+ \mu^-)$ $< 9.4 \times 10^{-10}$ at 95 % confidence level.