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

Abstract

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 collision data collected with the ATLAS detector in 2015 and 2016. Since the detector resolution in ${\mu }^{+}{\mu }^{-}$ 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 $\mathcal{B}(B_s^0\to {\mu }^{+}{\mu }^{-})=\left(3.2_{-1.0}^{+1.1}\right)\times 10^{-9}$ and an upper limit $\mathcal{B}(B^0\to {\mu }^{+}{\mu }^{-})<4.3\times 10^{-10}$ at 95% confidence level. The result is combined with the Run 1 ATLAS result, yielding $\mathcal{B}(B_s^0\to {\mu }^{+}{\mu }^{-})=\left(2.8_{-0.7}^{+0.8}\right)\times 10^{-9}$ and $\mathcal{B}(B^0\to {\mu }^{+}{\mu }^{-})<2.1\times 10^{-10}$ at 95% confidence level. The combined result is consistent with the Standard Model prediction within 2.4 standard deviations in the $\mathcal{B}(B^0\to {\mu }^{+}{\mu }^{-})$-$\mathcal{B}(B_s^0\to {\mu }^{+}{\mu }^{-})$ plane.

44 pages in total, author list starting page 28, 10 figures, 3 tables, submitted to JHEP. All figures including auxiliary figures are available at http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/BPHY-2018-09