New Production Mechanism for keV Sterile Neutrino Dark Matter by Decays of Frozen-In Scalars

TL;DR

This paper introduces a novel mechanism for producing keV sterile neutrino dark matter via decay of a frozen-in scalar, expanding the possibilities for dark matter genesis without requiring active-sterile mixing.

Contribution

It proposes a new scalar decay-based production mechanism for sterile neutrino dark matter that does not rely on thermal equilibrium or active-sterile mixing.

Findings

Successful dark matter abundance achieved in parameter space

Mechanism works without active-sterile neutrino mixing

Numerical Boltzmann analysis confirms viability

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.