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The rare electromagnetic J/ ^+^-^+^- decay is observed with a significance greatly exceeding the discovery threshold, using proton-proton collision data collected by the LHCb experiment during 2016-2018 at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 5. 4\, fb^-1. The rate of this decay is measured relative to that of the J/ ^+^- mode. Using the QED model for the four-muon decay in the efficiency estimation, its branching fraction is determined to be equation* B (J/ ^+^-^+^-) = (1. 130. 100. 050. 01) 10^-6, equation* where the uncertainties are statistical, systematic and due to the uncertainty on the branching fraction of the J/ ^+^- decay.
Aaij et al. (Thu,) studied this question.