Study of the rare decays of B⁰ₛ and B⁰ mesons into muon pairs using data collected during 2015 and 2016 with the ATLAS detector

A study of the decays B⁰ₛ → 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 B⁰ₛ-B⁰ mass difference, a single fit determines the signal yields for both decay modes. This results in a measurement of the branching fraction B(B0s→μ+μ−)=(3.2^+1.1_−1.0)×10−9B(B⁰ₛ→μ+μ−)=(3.2^+1.1_−1.0)×10⁻⁹ and an upper limit B(B⁰→μ⁺μ⁻)<4.3×10 at 95% confidence level. The result is combined with the Run 1 ATLAS result, yielding B(B⁰ₛ→μ⁺μ⁻)=(2.8^+0.8_−0.7)×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(B⁰ₛ →μ⁺μ⁻)plane.