Study of the rare decays of B_s⁰ and B⁰ mesons into muon pairs using data collected during 2015 and 2016 with the ATLAS detector

A study of the decays B_s⁰ → μ⁺μ⁻ and B⁰ → μ⁺μ⁻ has been performed using 26.3 fb⁻¹ of 13 TeV LHC proton-proton collision data collected with the ATLAS detector in 2015 and 2016. Since the detector resolution in μ⁺μ⁻ invariant mass is comparable to the B_s⁰-B⁰ mass difference, a single fit determines the signal yields for both decay modes. This results in a measurement of the branching fraction $B(B_s^0\to {\mu }^{+}{\mu }^{-})=\left({3.2}_{-1.0}^{+1.1}\right)\times {10}^{-9}$ and an upper limit $B(B^0\to {\mu }^{+}{\mu }^{-})<4.3\times {10}^{-10}$ at 95% confidence level. The result is combined with the Run 1 ATLAS result, yielding $B(B_s^0\to {\mu }^{+}{\mu }^{-})=\left({2.8}_{-0.7}^{+0.8}\right)\times {10}^{-9}$ and $B(B^0\to {\mu }^{+}{\mu }^{-})<2.1\times {10}^{-10}$ at 95% confidence level. The combined result is consistent with the Standard Model prediction within 2.4 standard deviations in the $B(B^0\to {\mu }^{+}{\mu }^{-})-B(B_s^0\to {\mu }^{+}{\mu }^{-})$ plane. .