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(Bs0→μ+μ−)=(3.2−1.0+1.1)×10−9 and an upper limit ℬ (B→ 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−)=(2.8−0.7+0.8)×10−9 and ℬ (B→ 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−) plane.[Figure not available: see fulltext.]