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

TL;DR

This paper models the thermalization process of energetic particles from heavy long-lived particles in the early universe, incorporating coherence effects and solving Boltzmann equations for the Standard Model particles.

Contribution

It introduces a detailed numerical framework for the thermalization cascade including gauge and fermion interactions, accounting for coherence effects in the early universe.

Findings

Thermal spectra depend on the ratio M/T and decay parameters.

Coherence effects significantly suppress splitting rates.

Numerical solutions reveal detailed thermalization dynamics.

Abstract

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.