Longevity problem of sterile neutrino dark matter

Sterile neutrino dark matter of mass O(1–10) keVO(1–10) keV decays into an active neutrino and an X-ray photon, and the non-observation of the corresponding X-ray line requires the sterile neutrino to be more long-lived than estimated based on the seesaw formula: the longevity problem. We show that, if one or more of the B–L Higgs fields are charged under a flavor symmetry (or discrete R symmetry), the split mass spectrum for the right-handed neutrinos as well as the required longevity is realized. We provide several examples in which the predicted X-ray flux is just below the current bound.