Dark Matter Freeze-out During Matter Domination

TL;DR

The paper investigates dark matter freeze-out during an early matter-dominated epoch driven by a long-lived, decoupled field $\phi$, where the expansion rate scales as $H\propto T^{3/2}$, in contrast to standard radiation domination. It introduces a dilution mechanism from $\phi$ decays that restores radiation domination before BBN, yielding a reduced DM abundance relative to the ordinary freeze-out and allowing different combinations of DM mass and annihilation cross section to reproduce the observed relic density. The authors derive a model-independent framework for the DM yield during matter domination, show how the relic density depends on the dilution factor $\zeta$ and the transition temperature $T_*$, and discuss viable parameter space under cosmological constraints, including $T_{\rm RH} \gtrsim 10$ MeV and potential implications for baryogenesis. Their results indicate that matter-dominated freeze-out can accommodate heavier DM with smaller couplings than in the standard scenario, providing a distinct avenue for DM phenomenology and experimental searches.

Abstract

We highlight the general scenario of dark matter freeze-out whilst the energy density of the universe is dominated by a decoupled non-relativistic species. Decoupling during matter domination changes the freeze-out dynamics, since the Hubble rate is parametrically different for matter and radiation domination. Furthermore, for successful Big Bang Nucleosynthesis the state dominating the early universe energy density must decay, this dilutes (or repopulates) the dark matter. As a result, the masses and couplings required to reproduce the observed dark matter relic density can differ significantly from radiation dominated freeze-out.

Figures (2)

• Figure 1: Contours of $\Omega_X h^2=0.1$ for matter (solid) and radiation (dashed) dominated freeze-out scenarios as we vary DM mass and coupling $\alpha$. Matter domination has two extra parameter freedoms, we fix $T_\star=10^6$ GeV and vary $\zeta$.
• Figure 2: Parameter space of matter dominated DM freeze-out assuming an $s$-wave annihilation cross section $\sigma_{0}\equiv\alpha^2/m_X^2$ for $\alpha=0.1$ with $T_\star\simeq m_\phi$, and thus $r\approx0.99$. We show contours of $T_{\rm RH}$ which give the observed DM relic density. The main constraints are that DM decouples prior to $\phi$ decays (shaded purple), but after the universe is $\phi$ matter dominated (red). We also require $T_{\rm FO}>T_{\rm RH}$ (green), in the yellow region we expect the instantaneous decay approximation to be less reliable. We shade regions in which the reheat temperature is low: for $T_{\rm RH}\lesssim10$ MeV BBN observables are disrupted (grey) and for $T_{\rm RH}\lesssim100$ GeV baryogenesis is challenging (blue).