Multiple Populations of Same Sterile Neutrino as Dark Matter

Abstract: Sterile neutrinos produced in the early Universe that mix with active neutrinos of the Standard Model are typically considered to consist of a single population resulting from one dominant production mechanism. We show that the same sterile neutrino species can naturally emerge with multiple population components, yielding a multi-modal relic momentum spectrum. We consider this with four distinct production scenarios: active-sterile non-resonant oscillations following resonant oscillations in the presence of a primordial lepton asymmetry, gravitational production through sterile neutrinogenesis from populations of evaporating primordial black holes, and heavy singlet Higgs or inflaton decays combined with non-resonant active-sterile oscillations or neutrinogenesis. We identify sterile neutrino mass ranges where a colder and a hotter population can be present with similar contributions and can also contribute non-negligibly to the dark matter relic abundance. We discuss some potential consequences of such a multi-population framework.