Z' portal dark matter from post-inflationary reheating: WIMPs, FIMPs, and UFOs

Abstract

We investigate the production of dark matter (DM) via a heavy $Z'$ mediator during the post-inflationary reheating epoch. In particular, we study production from three mechanisms which are smoothly connected to one another: WIMP-like freeze-out, FIMP-like freeze-in, and ultra-relativistic freeze-out (UFO). This is the first systematic study of $Z'$ portal DM which includes UFO. We find that much of the available parameter space for keV to TeV DM lies in the UFO regime for $ 1 \text{ TeV}\lesssim M_{Z'} \lesssim 1 \text{ PeV}$. When the mediator mass $M_{Z'}$ is greater than both the DM mass and the reheating temperature, UFO is a robust mechanism for producing cold DM. Although UFO DM is initially "hot" after freeze-out, it can easily become cold before structure formation if freeze-out occurs during post-inflationary reheating. Compared to standard freeze-in, UFO can accommodate significantly stronger interaction strengths (stronger couplings and/or smaller mediator masses).