Study of the rare decays of B s 0 and B 0 mesons into muon pairs using data collected during 2015 and 2016 with the ATLAS detector

A study of the decays B s 0 → μ + μ − and B 0 → 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 s 0 -B 0 mass difference, a single fit determines the signal yields for both decay modes. This results in a measurement of the branching fraction $ℬ\left({B}_{s}^{0}\to {\mu }^{+}{\mu }^{-}\right)=\left({3.2}_{-1.0}^{+1.1}\right)×{10}^{-9}$ and an upper limit $ℬ\left({B}^{0}\to {\mu }^{+}{\mu }^{-}\right)<4.3×{10}^{-10}$ at 95% confidence level. The result is combined with the Run 1 ATLAS result, yielding $ℬ\left({B}_{s}^{0}\to {\mu }^{+}{\mu }^{-}\right)=\left({2.8}_{-0.7}^{+0.8}\right)×{10}^{-9}$ and $ℬ\left({B}^{0}\to {\mu }^{+}{\mu }^{-}\right)<2.1×{10}^{-10}$ at 95% confidence level. The combined result is consistent with the Standard Model prediction within 2.4 standard deviations in the $ℬ\left({B}^{0}\to {\mu }^{+}{\mu }^{-}\right)-ℬ\left({B}_{s}^{0}\to {\mu }^{+}{\mu }^{-}\right)$ plane.