Ultrarelativistic Freeze-Out: A Bridge from WIMPs to FIMPs

We reexamine the case for dark matter (DM) produced by ultrarelativistic freeze-out (UFO). UFO is the mechanism by which standard model neutrinos decouple from the radiation bath in the early Universe at a temperature T_{d}≈1  MeV. This corresponds to chemical freeze-out without Boltzmann suppression, such that the freeze-out (decoupling) temperature T_{d} is much greater than m_{ν} and the neutrinos are therefore ultrarelativistic at freeze-out. While UFO has historically been rejected as a viable mechanism for DM production due to its association with hot DM and the accompanying incompatibility with ΛCDM, we show that when the approximation of instantaneous reheating after inflation is lifted, UFO can produce cold DM and account for the entire observed relic density in large regions of parameter space. In fact, DM with masses ranging from sub-eV to PeV scales can undergo UFO and be cold before structure formation, given only a simple perturbative, postinflationary reheating period prior to radiation domination. For some interactions, such as a contact interaction between the Higgs and DM scalars, there is a seamless transition between the weakly interacting massive particle and feebly interacting massive particle regimes that excludes UFO. However, for many other interactions, such as standard model fermions producing fermionic DM via a heavy scalar or vector mediator, the weakly interacting massive particle to feebly interacting massive particle transition occurs necessarily via a large intermediate regime corresponding to UFO. We characterize the general features of UFO in this Letter, while we supply a more detailed analysis in a companion paper. These results render the conventional mechanism for producing hot DM broadly compatible with cold DM.