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 $\varphi$ that undergoes energy diffusion, parametrized by the diffusion parameter $x$, the initial energy densities rate $\kappa \equiv {\rho }_{\varphi }/{\rho }_{\gamma }{|}_{\text{ini}}$, and the end-of-domination temperature $T_{\text{end}}$. We compare this UG+NSC scenario with standard NSC and $\Lambda$CDM cosmologies for WIMP Dark Matter (DM) production via the freeze-out mechanism. We find that energy diffusion reshapes the allowed $(m_{\chi },⟨\sigma v⟩)$ parameter space, where $m_{\chi }$ is the DM mass and $⟨\sigma v⟩$ is the thermally averaged annihilation cross section, opening regions otherwise excluded by DM overproduction in $\Lambda$CDM, and shifting the mass and cross-section ranges accessible to WIMP candidates depending on $x$, $\kappa$, $T_{\text{end}}$, and the barotropic index $\omega$ of $\varphi$. As a concrete application, we implement this framework for the Real Singlet Scalar WIMP and test its $(m_{\chi },{\lambda }_{HS})$ parameter space against current direct detection bounds from the LZ experiment, showing that energy diffusion opens previously unconstrained regions in the Higgs-portal coupling ${\lambda }_{HS}$ and thereby alters the detectability prospects of this benchmark model in future searches.

15 pages, 16 figures