NONEQUILIBRIUM PERTURBATION-THEORY FOR COMPLEX SCALAR FIELDS

Real-time perturbation theory is formulated for complex scalar fields away from thermal equilibrium in such a way that dissipative effects a rising from the absorptive parts of loop diagrams are approximately re summed into the unperturbed propagators. Low order calculations of phy sical quantities then involve quasiparticle occupation numbers which e volve with the changing state of the field system, in contrast with st andard perturbation theory, where these occupation numbers are frozen at their initial values. The evolution equation of the occupation numb ers can be cast approximately in the form of a Boltzmann equation. Par ticular attention is given to the effects of a nonzero chemical potent ial, and it is found that the thermal masses and decay widths of quasi particle modes are different for particles and antiparticles.