Updated studies on the sensitivity of FCC-ee to heavy neutral leptons coupling to electrons at the Z-pole run

This note has considered updated studies of the sensitivity of the Z-pole run at FCC-ee to Heavy Neutral Leptons with dominant couplings to the electron. Updated MC samples have been used to investigate an event selection requiring exactly two electrons, a veto on additional muons, jets and photons and exactly one reconstructed secondary vertex. Under the assumption that this selection would be background free the projected sensitivity is compared to previous FCC-ee projections (that assumed 100% signal efficiency and background-free search conditions under particular HNL scenarios). These results provide updated estimates of signal acceptances in displaced vertex signatures, and are consistent with previous projections accounting for the lack of inclusion of the semi-leptonic decay channel in this study, and exceed previous limits for some models with smaller couplings and higher HNL masses. Kinematic distributions of additional variables including angular distributions are presented for various benchmark HNL scenarios that could be further studied as a source of additional background discrimination or to characterise a potential signal. These results motivate further optimisation of the search strategy with larger MC background samples and calculations of the expected sensitivity when the di-electron and semileptonic channels are combined.