Freeze-in at stronger coupling

The predictivity of many nonthermal dark matter (DM) models is marred by the gravitational production background. Nevertheless, if the reheating temperature $T_R$ is low, the gravitationally produced relics can be diluted. We study the freeze-in dark matter production mechanism at temperatures below the dark matter mass. In this case, the coupling to the thermal bath has to be significant to account for the observed dark matter relic density. As a result, the direct DM detection experiments already probe such freeze-in models, excluding significant parts of parameter space.

Figure 1

Evolution of the individual terms appearing in the Boltzmann equation. Top: $T_R=10\mathrm{GeV}$, $m_s=223.5\mathrm{GeV}$, ${\lambda }_{hs}=0.05$. Bottom: $T_R=120\mathrm{GeV}$, $m_s=2.81\mathrm{TeV}$, ${\lambda }_{hs}=0.82$.

Figure 2

Parameter space of the Higgs portal freeze-in DM model. Along the curves, the correct DM relic abundance is reproduced. The curves are marked by the reheating temperature in GeV, while the purple line corresponds to thermal DM. The shaded area is excluded by the direct DM detection experiment LZ 2022. The neutrino background for direct detection is represented by the dashed line “$\nu$ fog”.

Figure 3

Evolution of the energy density components of the Universe for ${\mathrm{\Gamma }}_{\varphi }\sim {\mathrm{\Gamma }}_{\nu }$. ${\rho }_{\varphi }\propto a^{-3}$, ${\rho }_{\nu }\propto a^{-3/2}$, and ${\rho }_{\mathrm{SM}}\propto a^0$. Reheating occurs at the point where the curves meet.