Study of the rare decays of $B^0_s$ and $B^0$ into muon pairs with the ATLAS detector

A study of the decays $B^0_s \rightarrow \mu^+ \mu^-$ and $B^0 \rightarrow \mu^+ \mu^-$ 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 $B^0 \rightarrow \mu^+ \mu^- < 4.2 \times 10^{-10}$ at 95% confidence level. For $B^0_s$ the branching fraction $B^0_s \rightarrow \mu^+ \mu^- = (0.9^{+1.1}_{-0.8})\times 10^{-9}$ is measured. The results are consistent with the Standard Model expectation with a p-value of 4.8%, corresponding to 2.0 standard deviations.