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

A bstract 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 ^+^-) = (3. 2-₁. ₀^+1. 1) 10^-9 ℬ B s 0 → μ + μ − = 3. 2 − 1. 0 + 1. 1 × 10 − 9 and an upper limit B (B⁰ ^+^-) ℬ B 0 → μ + μ − 4. 3 × 10 − 10 at 95% confidence level. The result is combined with the Run 1 ATLAS result, yielding B (Bₛ⁰ ^+^-) = (2. 8-₀. ₇^+0. 8) 10^-9 ℬ B s 0 → μ + μ − = 2. 8 − 0. 7 + 0. 8 × 10 − 9 and B (B⁰ ^+^-) ℬ 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 (B⁰ ^+^-) -B (Bₛ⁰ ^+^-) ℬ B 0 → μ + μ − − ℬ B s 0 → μ + μ −