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

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 $33.6\text{}\text{}{\mathrm{fb}}^{-1}$. Normalizing to the $J/\psi \to {\mu }^{+}{\mu }^{-}$ decay mode leads to a branching fraction of $\left[{10.1}_{-2.7}^{+3.3}\left(\mathrm{stat}\right)±0.4\left(\mathrm{syst}\right)\right]×{10}^{-7}$, a value that is consistent with the standard model prediction.