Boosting Freeze-in through Thermalization

TL;DR

This paper demonstrates that thermalization and number-changing processes in the dark sector can significantly boost dark matter relic abundance, and provides a general method to compute this enhancement based on energy and entropy conservation.

Contribution

It introduces a universal approach to quantify the relic abundance enhancement due to dark sector thermalization, independent of specific particle physics models.

Findings

Thermalization can increase dark matter abundance by several orders of magnitude.

The enhancement can be calculated using energy and entropy conservation principles.

The method applies to both UV- and IR-dominated FIMP production.

Abstract

If the interaction rates between the visible and the dark sectors were never strong enough, the observed dark matter relic abundance could have been produced in the early Universe by non-thermal processes. This is what occurs in the so-called freeze-in mechanism. In the simplest version of the freeze-in paradigm, after dark matter is produced from the standard model thermal bath, its abundance is frozen and remains constant. However, thermalization and number-changing processes in the dark sector can have strong impacts, in particular enhancing the dark matter relic abundance by several orders of magnitude. Here we show that this enhancement can be computed from general arguments as the conservation of energy and entropy, independently from the underlying particle physics details of the dark sector. We also note that this result is quite general, and applies to FIMP production independently of being UV- or IR-dominated.