In many problems in particle cosmology, interaction rates are dominated by ${2}leftrightarrow{2}$ scatterings, or get a substantial contribution from them, given that ${1}leftrightarrow{2}$ and ${1}leftrightarrow{3}$ reactions are phase-space suppressed. We describe an algorithm to represent, regularize, and evaluate a class of thermal ${2}leftrightarrow{2}$ and ${1}leftrightarrow{3}$ interaction rates for general momenta, masses, chemical potentials, and helicity projections. A key ingredient is an automated inclusion of virtual corrections to ${1}leftrightarrow{2}$ scatterings, which eliminate logarithmic and double-logarithmic IR divergences from the real ${2}leftrightarrow{2}$ and ${1}leftrightarrow{3}$ processes. We also review thermal and chemical potential induced contributions that require resummation if plasma particles are ultrarelativistic.
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