New pathways to the relic abundance of vector-portal dark matter

We fully explore the thermal freeze-out 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' and dark matter chi is in the range 1 less than or similar to m(A')/m(chi) less than or similar to 2. In this region, 2 -> 2 and 3 -> 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 freeze-out 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 freeze-out. 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.