UCLA

We study two topics in this thesis: the halo-independent (HI) analysis method for dark matter (DM)-electron scattering and the phenomenology of sterile neutrinos ($\nu_s$s) produced in the evaporation of early Universe black holesWe demonstrate how the HI analysis can be applied to sub-GeV DM scattering off electrons for noble gas targets such as Xe and semiconductor targets such as Ge and Si. In the HI analysis method, properties of the local DM halo velocity distribution are inferred from direct DM detection data, which allows the comparison of different data sets without making any assumption on the uncertain local dark halo characteristics. This method had previously been developed for and applied only to DM scattering off nuclei. We additionally show that in-medium effects could significantly affect HI analysis results for semiconductor targets Ge and Si and thus are essential for proper inference of local DM halo characteristics from direct DM detection data. For $\nu_s$s as DM candidates, we discuss in detail the characteristics and phenomenology of $\nu_s$s that minimally couple only to active neutrinos and are produced in the evaporation of early Universe primordial black holes (PBHs), a process we call ``PBH sterile neutrinogenesis". Contrary to previously studied $\nu_s$ production mechanisms, this novel mechanism does not depend on the active-sterile mixing. The resulting $\nu_s$s have a distinctive spectrum and are produced with larger energies than in typical scenarios. This characteristic enables $\nu_s$s to be warm DM in the unusual $0.3$ MeV to $0.3$ TeV mass range if PBHs do not matter-dominate the Universe before evaporating. When PBHs matter-dominate before evaporating, the possible coincidence of induced gravitational waves associated with PBH evaporation and astrophysical X-ray observations from $\nu_s$ decays would constitute a distinct signature of our scenario.