Study of the Rare Decays of B{_{s}⁰} and B⁰ 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 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{{s}⁰} -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} → μ+μ−) = (3.2 {−1.0} ^{+1.1}) × 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{{s}⁰} → μ+μ−)=(2.8{{−0.7} ^{+0.8}}) × 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{_{s}⁰} → μ⁺μ⁻) plane. © 2019 CERN, for the ATLAS Collaboration