First observation of the rare $4\mu$ decay of the $\eta$ meson

The first observation of the double-Dalitz decay $\eta \rightarrow \mu^+\mu^-\mu^+\mu^-$ is reported with 2017 and 2018 data by the CMS experiment at the CERN LHC. A signal having a statistical significance well in excess of 5 standard deviations was identified using high-rate muon trigger data collected at a proton-proton collision center of mass energy of $13~\mathrm{TeV}$ corresponding to an integrated luminosity of $101~\mathrm{fb}^{-1}$. Using the $\eta \rightarrow \mu^+\mu^-$ decay as normalization, the branching fraction $\mathcal{B}(\eta \rightarrow \mu^+\mu^-\mu^+\mu^-) = \left(5.0 \pm 0.8~(\mathrm{stat}) \pm 0.7~(\mathrm{syst}) \pm 0.7~(\mathcal{B}) \right) \times 10^{-9}$ is measured, in agreement with theoretical estimates.