Study of the rare decay $J/\psi \to {\mu }^{+}{\mu }^{-}{\mu }^{+}{\mu }^{-}$

LHCb collaboration, R. Aaij, A. S. W. Abdelmotteleb +1099

The rare electromagnetic $J/\psi \to {\mu }^{+}{\mu }^{-}{\mu }^{+}{\mu }^{-}$ decay is observed with a significance greatly exceeding the discovery threshold, using proton-proton collision data collected by the…