Search for the dipole portal of heavy neutral leptons at future colliders

In this paper, we study the potential of future colliders to explore the parameter space of heavy neutral leptons (HNLs) through the dipole portal. We consider hadron colliders such as the LHC in the high luminosity phase and FCC-hh, and lepton colliders, such as FCC-ee. We consider various signatures for the HNLs, including the missing energy signature and displaced decays, and discuss the complementarity between the hadron and lepton colliders. We propose new selection rules which may significantly reduce the background events in FCC-ee. In particular, we find that thanks to a much clearer environment, FCC-ee may search for the HNLs with masses up to ≃ 30 GeV and proper lifetimes $c\tau_N ≳ 1$ cm, which is well beyond the reach of the experiments to be launched in the next decade.