  We fully explore the thermal freezeout histories of a vector-portal dark
matter model, in the region of parameter space in which the ratio of masses of
the dark photon $A^{\prime}$ and dark matter $\chi$ is in the range $1 \lesssim
m_{A^{\prime}}/m_{\chi} \lesssim 2$. In this region $2 \rightarrow 2$ and $3
\rightarrow 2$ annihilation processes within the dark sector, as well as
processes that transfer energy between the dark sector and the Standard Model,
play important roles in controlling the thermal freezeout of the dark matter.
We carefully track the temperatures of all species, relaxing the assumption of
previous studies that the dark and Standard Model sectors remain in thermal
equilibrium throughout dark matter freezeout. Our calculations reveal a rich
set of novel pathways which lead to the observed relic density of dark matter,
and we develop a simple analytic understanding of these different regimes. The
viable parameter space in our model provides a target for future experiments
searching for light (MeV-GeV) dark matter, and includes regions where the dark
matter self-interaction cross section is large enough to affect the small-scale
structure of galaxies.
