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 0s → 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 0s-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)×10−9 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(B0s→μ+μ−)=(2.8+0.8−0.7)×10−9 and B(B0→μ+μ−)