Observation of the J/ψ → μ⁺μ⁻μ⁺μ⁻ decay in proton-proton collisions at √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 [10.1 +3.3-2.7 (stat) ± 0.4 (syst)] × 10⁻⁷, a value that is consistent with the standard model prediction.