  We provide a quantum-field theoretic formulation of dressed particle dynamics
that systematically include particle production and scattering/decay processes
in the preheating era. Our approach is based on the so-called perturbation
theory in the Furry picture, in which coherent background fields (i.e.,
inflaton and the expanding Universe) are treated non-perturbatively whereas
interactions between dressed particles are taken into account perturbatively.
As application, we consider the instant preheating mechanism and compute the
number of produced particles explicitly, assuming that the coherent fields are
adiabatic. We find a novel non-perturbative particle-production mechanism,
which is kinematically forbidden within the conventional perturbative
calculation and produces particles exponentially efficiently than the
perturbative one does.
