NONEQUILIBRIUM QUANTUM-FIELDS IN THE LARGE-N EXPANSION

An effective action technique for the time evolution of a closed syste m eonsisting of one or more mean fields interacting with their quantum fluctuations is presented. By marrying large-N expansion methods to t he Schwinger-Keldysh closed time path formulation of the quantum effec tive action, causality of the resulting equations of motion is ensured and a systematic, energy-conserving and gauge-invariant expansion abo ut the quasiclassical mean field(s) in powers of 1/N developed. The ge neral method is exposed in two specific examples, O(N) symmetric scala r lambdaPHI4 theory and quantum electrodynamics (QED) with N fermion f ields. The lambdaPHI4 case is well suited to the numerical study of th e real time dynamics of phase transitions characterized by a scalar or der parameter. In QED the technique may be used to study the quantum n onequilibrium effects of pair creation in strong electric fields and t he scattering and transport processes in a relativistic e+e- plasma. A simple renormalization scheme that makes practical the numerical solu tion of the equations of motion of these and other field theories is d escribed.