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We explore the diverse cosmological histories of a dark sector that is connected to the Standard Model (SM) via a Dirac sterile neutrino. The dark sector consists of a complex scalar and a Dirac fermion dark matter (DM) candidate protected by a global U(1) stabilizing symmetry. Assuming the dark sector has negligible initial abundance and is populated from reactions in the SM thermal plasma during the radiation era, we show that the cosmological histories of the dark sector fall into four qualitatively distinct scenarios, each one characterized by the strengths of the portal couplings involving the sterile neutrino mediator. By solving Boltzmann equations, both semi-analytically and numerically, we explore these thermal histories and transitions between them in detail, including the time evolution of the temperature of the dark sector and the number densities of its ingredients. We also discuss how these various histories may be probed by cosmology, direct detection, indirect detection, collider searches, and electroweak precision tests. 

We investigate the influence of the reheating temperature of the visible sector on the freeze-in dark matter (DM) benchmark model for direct detection experiments, where DM production is mediated by an ultralight dark photon. Here, we consider a new regime for this benchmark: we take the initial temperature of the thermal Standard Model (SM) bath to be below the DM mass. The production rate from the SM bath is drastically reduced due to Boltzmann suppression, necessitating a significant increase in the portal coupling between DM and the SM to match the observed relic DM abundance. This enhancement in coupling strength increases the predicted DM-electron scattering cross section, making freeze-in DM more accessible to current direct detection experiments. Published by the American Physical Society2025