New production mechanism for keV sterile neutrino Dark Matter by decays of frozen-in scalars

Abstract

We propose a new production mechanism for keV sterile neutrino Dark Matter. In our setting, we assume the existence of a scalar singlet particle which never entered thermal equilibrium in the early Universe, since it only couples to the Standard Model fields by a really small Higgs portal interaction. For suitable values of this coupling, the scalar can undergo the so-called freeze-in process, and in this way be efficiently produced in the early Universe. These scalars can then decay into keV sterile neutrinos and produce the correct Dark Matter abundance. While similar settings in which the scalar does enter thermal equilibrium and then freezes out have been studied previously, the mechanism proposed here is new and represents a versatile extension of the known case. We perform a detailed numerical calculation of the DM production using a set of coupled Boltzmann equations, and we illustrate the successful regions in the parameter space. Our production mechanism notably can even work in models where active-sterile mixing is completely absent. * no primordial lepton asymmetry is present in the early Universe [17,18]. Indeed, a large enough primordial lepton asymmetry could lead to a resonant transition -the so-called Shi-Fuller mechanism [46]-producing a considerable amount of sterile neutrinos with a cooler non-thermal spectrum, in addition to the ones produced by the DW mechanism. In this way, some bounds could be evaded. On the other hand, in frameworks where the SM gauge group is extended, the sterile neutrinos could be charged non-trivially under the full gauge group and be sterile only with respect to SM interactions. In this case, although this is not compulsory [47], thermal production of keV neutrinos could be revived [48,49]. However, this mechanism would generically produce too much DM and by this overclose the Universe, thus requiring some dilution by the production of additional entropy [50]. Moreover, it could get into trouble with bounds from Big Bang nucleosynthesis [51].Probably the most versatile production mechanism from a particle physics point of view is the non-thermal production of keV sterile neutrinos by the decays of particles [52,53,54,55,56], in particular of singlet scalars. Examples of this production mechanism exists for the scalar being an inflaton [57,58] or a more general equilibrated scalar singlet particle [59,60]. This case is particularly interesting because it tends to lead to smaller bounds on the mass of the keV neutrino, a desirable feature, since keV-neutrinos with too large masses could be in danger with X-ray bound. For a recent collections of observational bounds from the non-observation of the decay into a light neutrino and a photon, N 1 → νγ, see Refs. [17,18,61] and references therein. 1 The aim of this paper is to study a variant of the scalar decay production mechanism discussed in Refs. [59,60]. The decisive point is that the scalar σ, which decays into the keV neutrinos, σ → N 1 N 1 , has to be efficiently produced in the early Universe, as otherwise it would ...

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