Sterile neutrino dark matter from freeze-in

A sterile neutrino is a well-motivated and widely studied dark matter (DM) candidate. The most straightforward realization of sterile neutrino DM, through the Dodelson-Widrow (DW) mechanism, is now ruled out by a combination of X-ray and Lyman-α measurements. An alternative production mechanism that is becoming increasingly popular in the literature is the freeze-in mechanism, involving frameworks where a feeble coupling to a particle — usually a scalar beyond the Standard Model — in the thermal bath results in a gradual accumulation of the sterile neutrino DM abundance. This paper reviews the various motivations for realizing such frameworks in the literature, their common characteristic features and phenomenological signatures.