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

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