Study of the rare decays of B^0s and B^0 mesons into muon pairs using data collected during 2015 and 2016 with the ATLAS detector

Abstract

A study of the decays B^0s → 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 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^0𝑠→𝜇+𝜇−)=(3.2+1.1−1.0)×10−9 and an upper limit (𝐵^0→𝜇+𝜇−)<4.3×10−10 at 95% confidence level. The result is combined with the Run 1 ATLAS result, yielding (𝐵^0𝑠→𝜇+𝜇−)=(2.8+0.8−0.7)×10−9 and (𝐵^0→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^0→𝜇+𝜇−)−(𝐵^0𝑠→𝜇+𝜇−) plane.