Direct detection experiments at the neutrino dipole portal frontier

Heavy sterile neutrinos are typically invoked to accommodate the observed neutrino masses, by positing a new Yukawa term connecting these new states to the neutrinos in the electroweak doublet. However, given our ignorance of the neutrino sector, we should explore additional interactions such sterile neutrinos may have with the Standard Model. In this paper, we study the dimension-5 operator which couples the heavy state to a light neutrino and the photon. We find that the recent XENON1T direct detection data can improve the limits on this neutrino dipole portal by up to an order of magnitude over previous bounds for tau neutrinos. Future direct detection experiments may be able to extend bounds for all three neutrino flavors down to the level probed by SN1987A.