Dark Matter Freeze-in from a Z ' Reheaton

Abstract We consider the Standard Model (SM) extended by a secluded U(1) D gauge sector encompassing Dirac fermion ( χ ) dark matter (DM), an abelian gauge boson Z ' and a SM-singlet complex-scalar field Φ, whose radial component drives cosmic inflation. When the Higgs portal coupling is small, the Z ' then acts as a “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 ' 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.