# Dark Matter Production during the Thermalization Era

## TL;DR

This paper analyzes non-thermal dark matter production during the universe's thermalization era, focusing on how different collision processes dominate based on the energy dependence of the cross section.

## Contribution

It provides a detailed calculation of dark matter abundance from various collision processes during reheating, highlighting the conditions under which each process dominates.

## Key findings

- High-energy particle collisions dominate for cross sections with n ≥ 4.
- Ambient plasma collisions are dominant for n ≤ 3 when reheating temperature is high.
- Mixed collisions are significant for n ≤ 2 at low reheating temperatures.

## Abstract

We revisit the non-thermal dark matter (DM) production during the thermalization and reheating era after inflation. The decay of inflaton produces high-energy particles that are thermalized to complete the reheating of the Universe. Before the thermalization is completed, DM can be produced from a collision between the high-energy particles and/or the ambient plasma. We calculate the DM abundance produced from these processes for the case where the cross section of the DM production is proportional to the $n$-th power of the center of mass energy. We find that the collision between the high-energy particles is almost always dominant for $n \gtrsim 4$ while it is subdominant for $n \lesssim 2$. The production from the ambient plasma is dominant when $n \lesssim 3$ and the reheating temperature is of the order of or larger than the DM mass. The production from a collision between the high-energy particle and the ambient plasma is important for $n \lesssim 2$ and the reheating temperature is much lower than the DM mass.