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

A study of the decays and has been performed using data corresponding to an integrated luminosity of 25 fb of 7 and 8 TeV proton–proton collisions collected with the ATLAS detector during the LHC Run 1. For the dimuon decay, an upper limit on the branching fraction is set at at 95 % confidence level. For , the branching fraction is measured. The results are consistent with the Standard Model expectation with a p value of 4.8 %, corresponding to 2.0 standard deviations.