Observation of the rare decay of the $\eta$ meson to four muons

CMS Collaboration

A search for the rare $\eta$ $\to$ ${\mu }^{+}{\mu }^{-}{\mu }^{+}{\mu }^{-}$ double-Dalitz decay is performed using a sample of proton-proton collisions, collected by the CMS experiment at the CERN LHC with hig…