Dynamic Freeze-In: Impact of Thermal Masses and Cosmological Phase Transitions on Dark Matter Production

TL;DR

This paper investigates how thermal masses and cosmological phase transitions affect dark matter production via freeze-in mechanisms, introducing three models that incorporate phase transition effects to enhance dark matter relic abundance calculations.

Contribution

It presents three novel freeze-in models that account for phase transition effects and thermal mass variations, improving the understanding of dark matter production in the early Universe.

Findings

Phase transitions can open or enhance dark matter production channels.

Thermal corrections and phase transition dynamics significantly impact relic abundance.

The models demonstrate the importance of detailed early Universe evolution in dark matter calculations.

Abstract

The cosmological abundance of dark matter can be significantly influenced by the temperature dependence of particle masses and vacuum expectation values. We illustrate this point in three simple freeze-in models. The first one, which we call kinematically induced freeze-in, is based on the observation that the effective mass of a scalar temporarily becomes very small as the scalar potential undergoes a second order phase transition. This opens dark matter production channels that are otherwise forbidden. The second model we consider, dubbed vev-induced freeze-in, is a fermionic Higgs portal scenario. Its scalar sector is augmented compared to the Standard Model by an additional scalar singlet, $S$, which couples to dark matter and temporarily acquires a vacuum expectation value (a two-step phase transition or `vev flip-flop'). While $\langle S \rangle \neq 0$, the modified coupling structure in the scalar sector implies that dark matter production is significantly enhanced compared to the $\langle S \rangle = 0$ phases realised at very early times and again today. The third model, which we call mixing-induced freeze-in, is similar in spirit, but here it is the mixing of dark sector fermions, induced by non-zero $\langle S \rangle$, that temporarily boosts the dark matter production rate. For all three scenarios, we carefully dissect the evolution of the dark sector in the early Universe. We compute the DM relic abundance as a function of the model parameters, emphasising the importance of thermal corrections and the proper treatment of phase transitions in the calculation.