Thermalization of matter decay products and non-thermal dark matter production in the early universe

Abstract

Heavy, out-of-equilibrium, decaying particles appear in many cosmological histories in the early universe. Our understanding of the late-time universe dictates that the energy injected into the radiation component of the energy budget of the universe should for the most part be thermalized and absorbed into the thermal radiation bath of particles. In this thesis we study the process of thermalization of these highly energetic decay products, dominated by the near-collinear splitting processes; the long quantum formation time in the latter calls for the inclusion of coherence effects, in the form of the LPM suppression. We introduce the main theoretical tools necessary for the study of the thermalization cascade of near-collinear splittings, and then move on to study this cascade for scenarios with a single species of non-Abelian gauge bosons, as well as all non-scalar particles of the Standard Model.