Multi-Species Thermalization Cascade of Energetic Particles in the Early Universe

Heavy long-lived particles are abundant in BSM physics and will, under generic circumstances, get to dominate the energy density of the universe. The resulting matter dominated era has to end before the onset of Big Bang Nucleosynthesis through the decay of the heavy matter component of mass $M$ into a thermal bath of temperature $T$. The process of thermalization primarily involves near-collinear splittings of energetic particles into two particles with lower energy. The correct treatment of these processes requires the inclusion of coherence effects which suppress the splitting rate. We write down and numerically solve the resulting coupled Boltzmann equations including all gauge bosons and fermions of the Standard Model (SM). We then comment on the dependence of the nonthermal spectra on the ratio $M/T$, as well as on the matter decay rate and branching ratios into various SM particles.