WIMP Freeze-Out in Diffusive Unimodular Gravity

Abstract

We study a non-standard cosmology (NSC) scenario within the Unimodular Gravity (UG) framework, sourced by a scalar field $ϕ$ that undergoes energy diffusion, parametrized by the diffusion parameter $x$, the initial energy densities rate $κ\equivρ_ϕ/ρ_γ|_{\text{ini}}$, and the end-of-domination temperature $T_{\text{end}}$. We compare this UG+NSC scenario with standard NSC and $Λ$CDM cosmologies for WIMP Dark Matter (DM) production via the freeze-out mechanism. We find that energy diffusion reshapes the allowed $(m_χ, \langleσv\rangle)$ parameter space, where $m_χ$ is the DM mass and $\langle σv \rangle$ is the thermally averaged annihilation cross section, opening regions otherwise excluded by DM overproduction in $Λ$CDM, and shifting the mass and cross-section ranges accessible to WIMP candidates depending on $x$, $κ$, $T_{\text{end}}$, and the barotropic index $ω$ of $ϕ$. As a concrete application, we implement this framework for the Real Singlet Scalar WIMP and test its $(m_χ, λ_{HS})$ parameter space against current direct detection bounds from the LZ experiment, showing that energy diffusion opens previously unconstrained regions in the Higgs-portal coupling $λ_{HS}$ and thereby alters the detectability prospects of this benchmark model in future searches.