$Z'$-Mediated Dark Matter with Low-Temperature Reheating

Abstract: We consider a simple extension of the standard model with fermionic dark matter (DM) and a $Z'$ gauge boson acting as a mediator. We also assume a scenario where cosmic reheating occurs at low temperatures due to the decay of a massive inflaton into standard model states. To follow the evolution of the background and the dark sector states, we implement the required Boltzmann equations in the code micrOMEGAs to explore both the freeze-out and freeze-in mechanisms. We determine the parameter space of the model that satisfies the relic density constraint under different assumptions for the reheating dynamics, and examine current constraints from DM direct detection, taking special care of the scenarios where DM was produced during the reheating era. Large regions of the parameter space favored by low-temperature reheating cases are already probed or will be within the reach of future experiments, both for the WIMP and the FIMP paradigms.