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 $n \gtrsim 4$ 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.
