Feynman rules for nonequilibrium field theory

Within the closed-time-path formalism, a set of real- and imaginary-time propagators is obtained, which are suitable for the perturbative evaluation of Green functions in nonequilibrium states of a scalar field theory. A generalized renormalization procedure, described in earlier works, allows for the partial resummation of absorptive parts of loop diagrams, so that finite quasiparticle lifetimes are incorporated in the unperturbed propagators. In this way, low-order calculations are made to reflect the time evolution of the nonequilibrium state, which is not correctly described by standard perturbation theory.