NONEQUILIBRIUM QUANTUM FIELDS WITH LARGE FLUCTUATIONS

We consider the nonequilibrium evolution of an O(N)–symmetric scalar quantum field theory using a systematic two–particle irreducible 1/N–expansion to next-to-leading order, which includes scattering and memory effects. The corresponding “full Kadanoff-Baym equations” are solved numerically without further approximations. This allows one to obtain a controlled nonperturbative description of far-from-equilibrium dynamics and the late-time approach to quantum thermal equilibrium. Employing, in addition, a first-order gradient expansion for the Wigner transformed correlators we derive kinetic equations. In contrast to standard descriptions based on loop expansions, our equations remain valid for nonperturbatively large fluctuations. As an application, we discuss the fluctuation dominated regime following parametric resonance in quantum field theory.