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arXiv · 2512.04229

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

Stephen E. Henrich·Yann Mambrini·Keith A. Olive
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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).

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Stephen E. Henrich, Yann Mambrini, Keith A. Olive. 2025-12-03. Z' portal dark matter from post-inflationary reheating: WIMPs, FIMPs, and UFOs. https://arxiv.org/abs/2512.04229

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