Dark matter production during the thermalization era

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 greater than or similar to 4 while it is subdominant for n less than or similar to 2. The production from the ambient plasma is dominant when n less than or similar to 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 less than or similar to 2 and the reheating temperature is much lower than the DM mass.