Abstract
We perform a detailed and illustrative study of the production of keV sterile neutrino Dark Matter (DM) by decays of singlet scalars in the early Universe. In the current study we focus on providing a clear and general overview of this production mechanism. For the first time we study all regimes possible on the level of momentum distribution functions, which we obtain by solving a system of Boltzmann equations. These quantities contain the full information about the production process, which allows us to not only track the evolution of the DM generation but to also take into account all bounds related to the spectrum, such as constraints from structure formation or from avoiding too much dark radiation. In particular we show that this simple production mechanism can, depending on the regime, lead to strongly non-thermal DM spectra which may even feature more than one peak in the momentum distribution. These cases could have particularly interesting consequences for cosmological structure formation, as their analysis requires more refined tools than the simplistic estimate using the free-streaming horizon. Here we present the mechanism including all concepts and subtleties involved, for now using the assumption that the effective number of relativistic degrees of freedom is constant during DM production, which is applicable in a significant fraction of the parameter space. This allows us to derive analytical results to back up our detailed numerical computations, thus leading to the most comprehensive picture of keV sterile neutrino DM production by singlet scalar decays that exists up to now.
Highlights
Despite great advances in our understanding, our Universe holds mysteries that are yet to be resolved
We have presented a fully comprehensive study of the production of keV sterile neutrino Dark Matter in the early Universe by singlet scalar decays
Our numerical solutions for the distribution functions have provided a comprehensive picture of where the observed Dark Matter abundance can be obtained in the large parameter space investigated, but they have allowed us to account for all bounds applicable
Summary
Despite great advances in our understanding, our Universe holds mysteries that are yet to be resolved. We concentrate on a candidate motivated by neutrino physics, a sterile neutrino νs of (typically) a mass of a few keV. While our natural guess would be for the sterile neutrino mass to be much larger, it is in reality not bound and there exist several consistent theoretical frameworks in which sterile neutrinos can be very light [12, 13]. The DW mechanism was at that time consistent with all bounds [15], we know that it is excluded by structure formation: it produces too hot DM [16]. Taking the sterile neutrino mass to larger values is not possible due to the X-ray bound on the DM decay νs → ν + γ, where ν is any active neutrino Taking the sterile neutrino mass to larger values is not possible due to the X-ray bound on the DM decay νs → ν + γ, where ν is any active neutrino (see Ref. [17] for a comprehensive discussion and Ref. [18] for probably the most recent collection of limits).
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