Sterile Neutrino as an Asymmetric Dark Matter

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-α, 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.

Figures (6)

• Figure 1: Evolution of the asymmetric sterile neutrino abundance $Y_{-}$ as a function of temperature for different values of the effective coupling $\mu$. The CP asymmetry parameter $\varepsilon$ and mediator mass $m_\phi$ are kept fixed.
• Figure 2: Evolution of the asymmetric sterile neutrino abundance $Y_{-}$ as a function of temperature for different values of the CP asymmetry parameter $\varepsilon$. The effective coupling $\mu$ and mediator mass $m_\phi$ are kept fixed.
• Figure 3: Evolution of the sterile neutrino yield $Y_{-}$ as a function of the temperature for different values of the mediator mass $m_\phi$. The other model parameters are fixed.
• Figure 4: Relic density $\Omega_{\rm DM} h^2$ as a function of the sterile neutrino mass $m_N$. The other model parameters are kept fixed. The linear behavior reflects the proportionality $\Omega_{\rm DM} h^2 \propto m_N Y_-^\infty$ characteristic of AFI production. The horizontal band indicates the observed DM relic abundance.
• Figure 5: Viable parameter space in the $(m_N,\mu)$ plane. The red diagonal band reproduces the observed DM relic density while satisfying the freeze-in condition and other phenomenological constraints. The vertical dashed line represents the Lyman-$\alpha$ lower bound on $m_N$. The allowed region corresponds to the overlap between the band and the region to the right of the Lyman-$\alpha$ constraint.