Infrared Effects of Ultraviolet Operators on Dark Matter Freeze-In

Dark matter (DM) that interacts too weakly with the Standard Model (SM) to reach full thermodynamic equilibrium can be still be created in significant amounts by rare SM collisions. This mechanism, called freeze-in, can proceed through a renormalizable connector operator with a very small coefficient, or a non-renormalizable connector operator suppressed by a large mass scale. In the latter non-renormalizable scenario, the dominant creation of DM particles typically occurs at the largest SM temperature attained during the radiation era (assuming a standard cosmological history), and for this reason it is referred to as ultraviolet freeze-in. We show that non-renormalizable operators can also contribute importantly to the DM density at lower temperatures down to below the mass of the DM particle. To do so, we compute the production, annihilation, and freeze-out of DM in a simple dark sector consisting of a massive Dirac fermion DM candidate coupled to a massless Abelian vector boson with the only connection to the SM through the fermionic Higgs portal operator. For a broad range of parameters in the theory, the dark sector is populated by ultraviolet freeze-in in the usual way, self-thermalizes to a dark temperature below that of the SM, and undergoes thermal freeze-out. We show that late residual freeze-in reactions during the freeze-out process can further populate the dark sector and increase the DM relic density beyond standard dark sector freeze-out.