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 parameterizing the general time-dependent Gaussian density matrix of the system. Yet, the evolution manifests both quantum decoherence and apparent irreversibility of energy flow from the coherent mean fields to fluctuating quantum modes. Using scalar QED as an example we show how this collisionless damping and decoherence may be understood as the result of {\em dephasing} of the rapidly varying fluctuations and particle production in the time varying mean field.