Study of the rare decays of B⁰_s 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_s⁰ → µ⁺ µ⁻) = ( 3.2⁺₋¹₁^._.¹₀ ) × 10⁻⁹ 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(B_s⁰ → µ⁺ µ⁻) = ( 2.8⁺₋⁰₀^._.⁸₇ )× 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(B_s⁰ → µ⁺ µ⁻) plane.