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Z' portal dark matter from post-inflationary reheating: WIMPs; FIMPs; and UFOs
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DOI：10.1088/1475-7516/2026/04/068.png)
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Abstract
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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 TeV ≲ MZ' ≲ 1 PeV. When the mediator mass MZ' 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).
Keywords:
dark matter
Z' mediator
freeze-out
freeze-in
reheating
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