Observation of the rare decay ημ^+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 (ημ^+μ^-e^+e^-) = (2. 4 0. 8) 10^-6, with the uncertainty including statistical and systematic sources as well as the B (ημ^+μ^-γ) uncertainty. This result is close to two orders of magnitude smaller than the existing limit, and is consistent with recent theoretical predictions.