Study of the rare decays of B0s and B0 into muon pairs from data collected during 2015 and 2016 with the ATLAS detector

A study of the decays $B_s^0 \to \mu^+\mu^−$ and $B^0 \to \mu^+\mu^−$ has been performed using $26.3$~fb$^{-1}$ of $13$ TeV LHC proton-proton collisions collected with the ATLAS detector in 2015 and 2016. For $B_s^0$, the branching fraction $BR(B_s^0 \to \mu^+\mu^−) = \left( 3.2^{+1.1}_{-1.0} \right) \times 10^{-9}$ is measured. For the $B^0$, an upper limit on the branching fraction is set at $BR(B^0 \to \mu^+\mu^−) < 4.3 \times 10^{-10}$ at $95\%$ confidence level. The result is combined with the full Run 1 ATLAS result, yielding $BR (B_s^0 \to \mu^+\mu^−) = \left( 2.8^{+0.8}_{-0.7} \right) \times 10^{-9}$ and $BR (B^0 \to \mu^+\mu^−) < 2.1 \times 10^{-10}$. The combined result is consistent with the Standard Model within 2.4 standard deviations.