Sensitivity of the FCC-ee to HNLs decaying to muon pairs and neutrinos

This note presents a study on the production of Heavy Neutral Leptons (HNLs) in Z decays at FCC-ee, through their mixing with the muon neutrino (ν_μ) and subsequent decay into μμν_μ, mediated by an off-shell Z or W boson. This channel is particularly clean, characterized by the presence of only two reconstructed muons in the detector and missing energy due to escaping neutrinos. The long-lived nature of the HNLs is exploited to effectively control the substantial Standard Model (SM) background arising from Z decays into muons, taus, and heavy flavors, as well as from the four-fermion process e⁺e⁻ → μμν_μν_μ. Limits on the signal are extracted under conservative assumptions and assuming that no background events survive the final selection criteria.