  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.
