Abstract
Sterile neutrinos with a mass of a few keV can serve as cosmological warm dark matter. We study the production of keV sterile neutrinos in the early universe from the decay of a frozen-in scalar. Previous studies focused on heavy frozen-in scalars with masses above the Higgs mass leading to a hot spectrum for sterile neutrinos with masses below 8-10 keV. Motivated by the recent hints for an X-ray line at 3.55 keV, we extend the analysis to lighter frozen-in scalars, which allow for a cooler spectrum. Below the electroweak phase transition, several qualitatively new channels start contributing. The most important ones are annihilation into electroweak vector bosons, particularly W-bosons as well as Higgs decay into pairs of frozen-in scalars when kinematically allowed.
Highlights
Couple extremely weakly to the thermal bath of SM particles
We study the production of keV sterile neutrinos in the early universe from the decay of a frozen-in scalar
The dark matter (DM) production mechanism works without the Z4 symmetry, but the parameters have to be appropriately tuned to decouple the keV sterile neutrino and right-handed neutrinos in the usual way
Summary
We consider the sterile neutrino production in the early universe. If the keV sterile neutrino gets into thermal equilibrium with SM particles, it will be overproduced and will overclose the universe after freeze-out. First a feebly-coupled scalar field, σ, is produced via a tiny Higgs portal coupling, λHφ, which has to be small enough such that σ is always out of thermal equilibrium This scalar subsequently decays into keV sterile neutrinos. See figure 1a for a typical evolution of the abundances of the keV sterile neutrino (red) and the scalar σ (blue) for a frozen-in scalar with a mass larger than the mass of the Higgs. Note that the production of the keV sterile neutrino is dominated by Higgs decay and very weakly depends on the physics above the EW phase transition. As the coupling λHφ is extremely small, the contribution to the invisible decay width of the Higgs is negligibly small
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