Observation of the J/psi -> mu+ mu- mu+ mu- decay in proton-proton collisions at sqrt(s) = 13 TeV

The 𝐽/𝜓 →𝜇+⁢𝜇−⁢𝜇+⁢𝜇− 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/𝜓 →𝜇+⁢𝜇− decay mode leads to a branching fraction of [10.1+3.3−2.7⁢(stat) ±0.4⁢(syst)] ×10−7, a value that is consistent with the standard model prediction.