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

TL;DR

This paper investigates a secluded $U(1)_D$ gauge sector with a $Z^\prime$ reheaton that facilitates dark matter freeze-in, reheating, and generates gravitational waves, linking particle physics with cosmological observations.

Contribution

It introduces a model where a $Z^\prime$ acts as a reheaton and enables dark matter freeze-in, incorporating non-perturbative effects and gravitational wave predictions.

Findings

Viable parameter space for dark matter freeze-in via $Z^\prime$ decays.

Reheating temperature below 10 TeV with gauge portal interactions.

Gravitational wave signals from preheating and inflation can probe the reheating process.

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.