Two portals to GeV sterile neutrinos: Dipole versus mixing

Massive sterile neutrinos, also known as heavy neutral leptons, can have a mixing with active neutrinos, 0, as well as a dipole coupling to the photon, d. We study the interplay between these two portals, considering the production from meson decays of sterile neutrinos with mass 0.1 GeV <= MN <= 10 GeV, at beam-dump facilities such as NA62 and SHiP, and at the FASER2 experiment. These sterile neutrinos can be long-lived and decay into a photon in a distant detector, via the dipole operator. We find that all these experiments will be sensitive to values of d which are presently unconstrained. The experimental reach varies strongly with the mass MN and the mixing 0, and one observes specific correlations with the flavour of active neutrinos. The SHiP experiment will mark a jump in sensitivity: (i) it will probe a sterile dipole as small as d similar to 10-8 GeV-1, thus testing new physics well above the electroweak scale; (ii) it may detect the active-sterile dipole to the level predicted by electroweak loops, if 0 is close to the present bound.