Study of the rare decays of B⁰s and B⁰ into muon pairs from data collected during the LHC Run 1 with the ATLAS detector

A study of the decays B⁰_s→μ⁺μ⁻ and B⁰→μ⁺μ⁻ has been performed using data corresponding to an integrated luminosity of 25 fb⁻¹ of 7 and 8 TeV proton–proton collisions collected with the ATLAS detector during the LHC Run 1. For the B⁰ dimuon decay, an upper limit on the branching fraction is set at B(B⁰→μ⁺μ⁻) <4.2 × 10⁻¹⁰ at 95 % confidence level. For B⁰_s, the branching fraction B(B⁰_s→ μ⁺μ⁻) = (0.9^+1.1_−0.8) ×10⁻⁹ is measured. The results are consistent with the Standard Model expectation with a p value of 4.8 %, corresponding to 2.0 standard deviations.