  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 $^{+3.3}_{-2.7}$
(stat) $\pm$ 0.4 (syst)] $\times$ 10$^{-7}$, a value that is consistent with
the standard model prediction.
