Observation of the J/$\psi$ $\to$ ${\mu }^{+}{\mu }^{-}{\mu }^{+}{\mu }^{-}$ decay in proton-proton collisions at $\sqrt{s}$ = 13 TeV

The J/$\psi$ $\to$ ${\mu }^{+}{\mu }^{-}{\mu }^{+}{\mu }^{-}$ 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/$\psi$ $\to$ ${\mu }^{+}{\mu }^{-}$ decay mode leads to a branching fraction [10.1 ${}_{-2.7}^{+3.3}$ (stat) $\pm$ 0.4 (syst)] $\times$ 10${}^{-7}$, a value that is consistent with the standard model prediction.