DISSIPATION AND DECOHERENCE IN MEAN-FIELD THEORY

The time evolution of a closed system of mean fields and fluctuations is Hamiltonian, with the canonical variables parametrizing the general time-dependent Gaussian density matrix of the system. Yet, the evolut ion manifests both quantum decoherence and apparent irreversibility of energy flow from the coherent mean fields to fluctuating quantum mode s. Using scalar QED as an example, we show how this collisionless damp ing and decoherence may be understood as the result of dephasing of th e rapidly varying fluctuations and particle production in the time var ying mean field.