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→ μ⁺μ⁻)=(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→ μ⁺μ⁻)<4.3× 10⁻¹⁰ 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→ μ⁺μ⁻)=(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→ μ⁺μ⁻)<2.1× 10⁻¹⁰ 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→ μ⁺μ⁻)-B(Bₛ⁰→ μ⁺μ⁻) ℬ B 0 → μ + μ − − ℬ B s 0 → μ + μ −