Study of the rare decays of $B^0$ and $B^0_s$ to muon pairs with the ATLAS detector at LHC Run-­1

A study of the decays $B^0\to \mu^+\mu^-$ and $B^0_s\to \mu^+\mu^-$ has been performed using 25~fb$^{-1}$ of integrated luminosity of proton-proton collisions collected with the ATLAS detector during LHC Run 1. For $B^0$, an upper limit on the branching ratio is set at ${\cal B}(B^0 \to \mu^+\mu^-) < 4.2 \times 10^{-10}$ at 95% confidence level. For $B^0_s$, the branching ratio ${\cal B}(B^0_s \to \mu^+\mu^-) = \left( 0.9^{+1.1}_{-0.8} \right) \times 10^{-9}$ is obtained. The result is consistent with the Standard Model expectation with a p-value of 4.8%.