Detectable Dark Matter Freeze-in via Low Reheating

Abstract

This work focuses on freeze-in production of dark matter (DM) during low reheating, where the latter arises from the decay of inflatons leading to a non-adiabatic matter-dominated era. Under freeze-in, DM comoving yield is inversely proportional to Hubble, and during matter domination, as Hubble increases, the coupling, keeping the DM interaction with SM, should be enhanced to have relic density $Ω_{DM}\simeq0.12$, making DM potentially vulnerable to direct detection searches. A renormalizable Yukawa-like Lagrangian is considered for DM interaction with a singlet electron $e$, and a BSM parent particle $P$. With the decay being the dominant freeze-in production channel, the required coupling enhancement is calculated with the aid of freeze-in Boltzmann equation. Dealing with freeze-in, the higher coupling could thermalize the feebly interacting particles, and this places an upper bound on coupling. The coupling has been found to be sensitive to the vacuum energy of the inflaton during the inflationary phase. Keeping this vacuum energy at the upper bound from {\tt planck}, the model fails to feature any detectable direct detection signals even at a reheating temperature of BBN. The findings of this thesis then show that, only under low reheating and low inflationary vacuum energy, the DM-electron cross sections become high enough to be detectable in near-future direct detection searches. However, the proposed model becomes non-perturbative near the direct detection exclusion limits.