NONEQUILIBRIUM DYNAMICS OF SYMMETRY-BREAKING IN LAMBDA-PHI(4) THEORY

The time evolution of O(AT) symmetric lambda(Phi(2))(2) scalar field t heory is studied in the large N limit. In this limit the [Phi(i)] mean field and two-point correlation function [Phi(i) Phi(j)] evolve toget her as a self-consistent closed Hamiltonian system, characterized by a Gaussian density matrix. The static part of the effective Hamiltonian defines the true effective potential U-eff for configurations far Fro m thermal equilibrium. Numerically serving the time evolution equation s for energy densities corresponding to a quench in the unstable spino dal region, we find results quite different from what might be inferre d from the equilibrium free energy potential F. Typical time evolution s show effectively irreversible energy flow from the coherent mean fie lds to the quantum fluctuating modes, due to the creation of massless Goldstone bosons near threshold. The plasma frequency and collisionles s damping rate of the mean fields are calculated in terms of the parti cle number density by a linear response analysis and compared with the numerical results. Dephasing of the fluctuations leads also to the gr owth of an effective entropy and the transition from quantum to classi cal behavior of the ensemble. In addition to casting some light on fun damental issues of nonequilibrium quantum statistical mechanics, the g eneral framework presented in this work may be applied to a study of t he dynamics of second order phase transitions in a wide variety of Lan dau-Ginsburg systems described by a scalar order parameter.