Study of the rare decays of $B_s^0$ and $B^0$ into muon pairs from data collected during the LHC Run 1 with the ATLAS detector

A study of the decays $B^0\to {\mu }^{+}{\mu }^{-}$ and $B_s^0\to {\mu }^{+}{\mu }^{-}$ has been performed using data corresponding to an integrated luminosity of $25$ fb${}^{-1}$ of $7$ TeV and $8$ TeV proton–proton collisions collected with the ATLAS detector during the LHC Run 1. For $B^0$, an upper limit on the branching fraction is set at $\mathcal{B}(B^0\to {\mu }^{+}{\mu }^{-})<4.2\times {10}^{-10}$ at $95\mathrm{\%}$ confidence level. For $B_s^0$, the branching fraction ${\cal B}(B^0_s \to \mu^+\mu^-) = \left( 0.9^{+1.1}_{-0.8} \right) \times 10^{-9}$ is measured. The results are consistent with the Standard Model expectation with a $p$-value of $4.8\mathrm{\%}$, corresponding to $2.0$ standard deviations.