Sterile neutrino dark matter from right-handed neutrino oscillations

We study a scenario where sterile neutrino (either warm or cold) dark matter (DM) is produced through (nonresonant) oscillations among right-handed neutrinos (RHNs) and can constitute the whole DM in the Universe, in contrast to the conventional sterile neutrino production through its mixing with the left-handed neutrinos. The lightest RHN can be sterile neutrino DM whose mixing with left-handed neutrinos is sufficiently small while heavier RHNs can have non-negligible mixings with left-handed neutrinos to explain the neutrino masses by the seesaw mechanism. We also demonstrate that, in our scenario, the production of sterile RHN DM from the decay of a heavier RHN is subdominant compared with the RHN oscillation production due to the x-ray and small-scale structure constraints.