Observation of the rare decay $η$$→μ⁺μ⁻e⁺e⁻

A first observation of the rare decay $η$ → μ⁺μ⁻e⁺e⁻ is reported by the CMS Collaboration at the CERN LHC. The result is based on a proton-proton collision data sample at √s = 13.6 TeV corresponding to an integrated luminosity of 38.0 fb⁻¹, acquired in 2022 using a high-rate dimuon trigger. Using the $η$ →μ^+μ^-γ$ decay channel for normalization, the branching fraction is measured to be $B$($η→μ^+μ^-e^+e^-$) = (2.4 $±$ 0.8)$×$ 10$⁻⁶$, with the uncertainty including statistical and systematic sources as well as the $B$($η→μ^+μ^-γ$) uncertainty. This result is close to two orders of magnitude smaller than the existing limit, and is consistent with recent theoretical predictions.