Freeze-in at Low Reheating and Direct Detection of Fermion Dark Matter

TL;DR

This work analyzes a low-$T_{rh}$ freeze-in scenario for fermionic dark matter $\psi$ coupled to a pseudoscalar mediator $s$ within a minimal Higgs-portal framework. By solving the Boltzmann evolution with $s$ in thermal equilibrium and $\psi$ as a Feebly Interacting Massive Particle (FIMP), the authors derive the DM yield from $s\to\psi\bar{\psi}$ decays, assess thermalization/non-thermalization bounds for $s$ and $\psi$, and compute a loop-induced DM–Higgs coupling that governs direct detection. The relic density can receive a subdominant Super-WIMP contribution, but freeze-in dominates in the examined region; the resulting spin-independent cross section is tested against current LZ limits and future DARWIN prospects. The results demonstrate tangible direct-detection sensitivity in low-$T_{rh}$ freeze-in scenarios and outline extensions to related two-field models and symmetry structures, highlighting complementary pathways to probe feebly interacting DM.

Abstract

We investigate a low-reheating-temperature freeze-in scenario within a minimal model of fermionic dark matter interacting through a pseudoscalar mediator. In this setup, dark matter is produced via the decay of the pseudoscalar, which remains in thermal equilibrium with the Standard Model bath. We derive the thermalization and non-thermalization conditions for the new fields and obtain the corresponding direct-detection constraints and projections on the model based on LZ and DARWIN experiments, respectively.

Figures (5)

• Figure 1: Minimum temperature required to thermalize the pseudoscalar mediator $s$ as a function of the Higgs-portal coupling $\lambda_{hs}$ and the pseudoscalar mass $m_s$. The red region indicates the bounds from Higgs invisible decays ATLAS:2022yvh. These results were obtained using micrOMEGAs.
• Figure 2: Thermalization bounds for different values of $n = m_s/m_\psi$ (thermalization occurs above the gray lines) and contours of the correct relic abundance via freeze-in for different reheating temperatures $T_\text{rh}$ (red lines).
• Figure 3: (a) Leading contribution to the DM--Nucleon scattering cross section. (b) Contours of $\sigma_\text{SI}$ in the $m_\psi$--$y_p$ plane for $m_s = 2 m_\psi$ (dashed blue lines). The red region corresponds to the current exclusion limits from LZ LZ:2024zvo and the black line to the projected sensitivity of DARWIN DARWIN:2016hyl.
• Figure 4: Region of parameter space showing the contours of correct relic abundance (blue lines), exclusion by LZ (dark red) and DARWIN projections (light red), and the region where $\psi$ remains out-of-thermal equilibrium (light blue region). The gray region is not part of our study since $m_s < 2m_\psi$.
• Figure 5: One-loop contribution to the effective DM--Higgs coupling.