Observation of the Rare Decay of the Eta Meson to a Muon Pair and an Electron Pair with the CMS Experiment

The rare decay of the $\eta$ meson to a muon pair and an electron pair is observed for the first time, by the CMS collaboration at the CERN LHC. The data is collected with a special double-muon trigger used in 2022, corresponding to 38 fb$^{-1}$ of integrated luminosity. The process is observed with a significance well in excess of 5 standard deviations. A measurement of the branching fraction of this decay mode is also performed, using the $eta$ meson to 2 muons decay mode as normalization channel.