Sterile Neutrino Self-Interactions in Cosmology: From Dark Matter to the Baryon Asymmetry

Sterile neutrinos represent a compelling and well-motivated extension of the Standard Model, capable of addressing fundamental open questions such as the origin of dark matter and the baryon asymmetry of the Universe. However, the minimal scenarios for sterile neutrino dark matter and freeze-in leptogenesis face significant challenges. On the one hand, keV-scale sterile neutrino dark matter produced through oscillations via the Dodelson–Widrow mechanism is ruled out by a combination of X-ray searches and structure formation constraints. On the other hand, the minimal freeze-in leptogenesis scenario requires a high degree of degeneracy among the right-handed neutrinos, motivating the exploration of alternative frameworks. To address these limitations, this thesis investigates the role of sterile neutrino self-interactions in both contexts. In the dark matter study, we consider sterile neutrinos interacting via a singlet scalar, the latter with masses ranging from a few keV to a few GeV. These new interactions enable a self-accelerating production of sterile neutrinos: by allowing the sterile sector to draw additional energy from the active sector, the resulting sterile neutrino abundance can be significantly boosted compared to the vanilla Dodelson–Widrow scenario for the same mixing angle. Within this framework, we identify several distinct production regimes exhibiting diverse phenomenology, from partial thermalization for low and intermediate mediator masses to resonant production for heavier mediators. We demonstrate that significant regions of parameter space open up, highlighting promising regions for future observations. The second study, focusing on freeze-in leptogenesis, considers a pair of sterile neutrino flavours with self-interactions mediated by a singlet scalar via a Yukawa-like coupling. The new interactions modify the dynamics in two key ways: they lead to an additional rate of production and destruction of the sterile neutrinos and to the generation of a new contribution to the effective potential. Assuming that the Yukawa and mass matrices are not diagonal in the same basis, we find that the new contributions to the sterile effective potential can enhance the produced baryon asymmetry by several orders of magnitude, substantially relaxing the mass degeneracy constraints of the minimal scenario.