Probing right-handed neutrinos dipole operators

We consider the minimal see-saw extension of the Standard Model with two right-handed singlet fermions N1,2 with mass at the GeV scale, augmented by an effective dipole operator between the sterile states. We firstly review current bounds on this effective interaction from fixed-target and collider experiments, as well as from astrophysical and cosmological observations. We then highlight the prospects for testing the decay N2 → N1γ induced by the dipole at future facilities targeting long-lived particles such as ANUBIS, CODEX-b, FACET, FASER 2, MAPP and SHiP.