Publication
The J/psi -> mu(+)mu(-)mu(+)mu(-) decay has been observed with a statistical significance in excess of five standard deviations. The analysis is based on an event sample of proton-proton collisions at a center-of-mass energy of 13 TeV, collected by the CMS experiment in 2018 and corresponding to an integrated luminosity of 33.6 fb(-1). Normalizing to the J/psi -> mu(+)mu(-) decay mode leads to a branching fraction of [10.1(-2.7)(+3.3)(stat) +/- 0.4(syst)] x 10(-7), a value that is consistent with the standard model prediction.