Study of the rare decay J/ψ → μ+μ−μ+μ−

The rare electromagnetic J/ψ → µ +µ −µ +µ − decay is observed with a signifcance 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/ψ → µ +µ − mode. Using the QED model for the four-muon decay in the efciency estimation, its branching fraction is determined to be B(J/ψ → µ +µ −µ +µ −) = (1.13 ± 0.10 ± 0.05 ± 0.01) × 10−6 , where the uncertainties are statistical, systematic and due to the uncertainty on the branching fraction of the J/ψ → µ +µ − decay.