  Boltzmann equations and their matrix valued generalisations are commonly used
to describe nonequilibrium phenomena in cosmology. On the other hand, it is
known that in gauge theories at high temperature processes involving many
quanta, which naively are of higher order in the coupling, contribute to the
relaxation rate at leading order. How does this accord with the use of single
particle distribution functions in the kinetic equations? When can these
effects be parametrised in an effective description in terms of quasiparticles?
And what is the kinematic role of their thermal masses? We address these
questions in the framework of nonequilibrium quantum field theory and develop
an intuitive picture in which contributions from higher order processes are
parametrised by the widths of resonances in the plasma. In the narrow width
limit we recover the quasiparticle picture, with the additional processes
giving rise to off-shell parts of quasiparticle propagators that appear to
violate energy conservation. In this regime we give analytic expressions for
the scalar and fermion nonequilibrium propagators in a medium. We compare the
efficiency of decays and scatterings involving real quasiparticles, computed
from analytic expressions for the relaxation rates via trilinear scalar and
Yukawa interactions for all modes, to off-shell contributions and find that the
latter can be significant even for moderate widths. Our results apply to
various processes including thermal production of particles from a plasma,
dissipation of fields in a medium and particle propagation in dense matter. We
discuss cosmological implications, in particular for the maximal temperature
achieved during reheating by perturbative inflaton decay.
