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 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/ψ → μ⁺μ⁻ mode. Using the QED model for the four-muon decay in the efficiency estimation, its branching fraction is determined to be {equation*} {B}(J/ψ → μ^+μ^-μ^+μ^-) = (1.13±0.10±0.05±0.01)× 10⁻⁶, {equation*} where the uncertainties are statistical, systematic and due to the uncertainty on the branching fraction of the J/ψ → μ⁺μ⁻ decay.