Temperature evolution in the Early Universe and freeze-in at stronger coupling

Abstract: Dark matter freeze-in at stronger coupling is operative when the Standard Model (SM) bath temperature never exceeds the dark matter mass. An attractive feature of this scenario is that it can be probed by direct detection experiments as well as at the LHC. In this work, we show how the mechanism can be realized in a simple UV complete framework, emphasizing the role of the maximal temperature of the SM thermal bath. We demonstrate that the maximal temperature can coincide with the reheating temperature or be close to it such that dark matter production is always Boltzmann-suppressed. This possibility is realized, for example, if the inflaton decays primarily into feebly interacting right-handed neutrinos, which subsequently generate the SM thermal bath. In this case, the SM sector temperature remains constant over cosmological times prior to reheating.