Induced resonance makes light sterile neutrino dark matter cool

We describe two new generation mechanisms for dark matter (DM) composed of sterile neutrinos with O(1) keV mass. The model contains a light scalar field that coherently oscillates in the early Universe and modulates the Majorana mass of the sterile neutrino. In a region of model parameter space, the oscillations between active and sterile neutrinos are resonantly enhanced. This mechanism allows us to produce sterile neutrino DM with a small mixing angle with active neutrinos, thus evading the x-ray constraints. At the same time, the spectrum of produced DM is much cooler than in the case of ordinary oscillations in plasma, opening a window of lower mass DM, which is otherwise forbidden by structure formation considerations. In other regions of the model parameter space, where the resonance does not appear, another mechanism can operate: a large field suppresses the active-sterile oscillations, but instead sterile neutrinos are produced by the oscillating scalar field when the effective fermion mass crosses zero. In this case, the DM component is cold, and even the 1 keV neutrino is consistent with the cosmic structure formation.