Study of the rare decays of Bs⁰ and B⁰ mesons into muon pairs using data collected during 2015 and 2016 with the ATLAS detector

Abstract

A study of the decays Bs⁰ → 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 Bs⁰ -B⁰ mass difference, a single fit determines the signal yields for both decay modes. This results in a measurement of the branching fraction B(Bs⁰ → µ⁺µ⁻) = 3.2 +1.1 −1.0x10⁻⁹ and an upper limit B(B⁰ → µ⁺µ⁻) < 4.3 × 10⁻¹⁰ at 95% confidence level. The result is combined with the Run 1 ATLAS result, yielding B(Bs⁰ → µ⁺µ⁻) = 2.8 +0.8 −0.7x 10⁻⁹ and B(B⁰ → µ+µ⁻) < 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(B⁰ → µ⁺µ⁻)- B(Bs⁰ → µ⁺µ⁻) plan