Dark Matter Freeze-in from a $Z^{\prime }$ Reheaton

Abstract

We consider the Standard Model (SM) extended by a secluded $U(1{)}_D$ gauge sector encompassing a Dirac fermion ($\chi$) dark matter (DM), an abelian gauge boson $Z^{\prime }$ and a SM-singlet complex-scalar field $\Phi$, whose radial component drives cosmic inflation. When the Higgs portal coupling is small, the $Z^{\prime }$ then acts as a {\it ``reheaton''}, dominating the energy budget of the Universe before finally yielding the SM bath, with reheating temperature $<O(10)$ TeV, through the gauge portal interaction. We explore the possibility that DM freezes-in via non-thermal $Z^{\prime }$ decays before reheating ends, giving rise to substantial viable parameter space. We account for non-perturbative effects, relevant during the initial stages of reheating, using lattice simulations. We additionally show how the cosmological gravitational wave (GW) background produced by preheating and inflation allow for a direct probe of the reheating mechanism.

Comments: 32 pages, 8 figures, references added; matches version published in JCAP