Sterile Neutrino as an Asymmetric Dark Matter

TL;DR

This paper introduces a minimal, predictive model for asymmetric sterile neutrino dark matter produced via freeze-in, which naturally reproduces observed relic abundance and is consistent with cosmological constraints.

Contribution

It presents a novel asymmetric freeze-in framework with a gauge-singlet Dirac sterile neutrino, scalar mediator, and auxiliary fermion, providing a self-consistent dark matter production mechanism.

Findings

Relic abundance matches observations without thermal equilibrium.

The momentum distribution of dark matter is colder than thermal spectra.

Parameter space is constrained by multiple cosmological and experimental bounds.

Abstract

We propose a minimal and predictive framework for asymmetric sterile neutrino dark matter (DM) produced via freeze-in. The Standard Model (SM) is extended by a gauge-singlet Dirac sterile neutrino carrying a conserved dark charge, a real scalar mediator, and an auxiliary singlet fermion. DM is generated through the out-of-equilibrium decay of the mediator, which simultaneously produces a particle{antiparticle asymmetry in the sterile sector controlled by a CP-violating parameter. We show that the observed relic abundance can be naturally reproduced without thermal equilibration with the SM plasma. The resulting non-thermal momentum distribution is colder than a thermal Fermi{Dirac spectrum, ensuring consistency with structure formation constraints. Combining relic density, Lyman-{\alpha}, Higgs invisible decay, and big bang nucleosynthesis (BBN) bounds, we identify correlated and predictive regions of the parameter space characterized by non-trivial relations among the sterile neutrino mass and the decay parameters. This scenario provides a self-consistent realization of Dirac asymmetric sterile neutrino DM within an asymmetric freeze-in (AFI) framework, offering a constrained and testable alternative to conventional production mechanisms.