  We analyze the dynamics of dissipation and relaxation in the unbroken and
broken symmetry phases of scalar theory in the nonlinear regime for large
initial energy densities, and after linear unstabilities (parametric or
spinodal) are shut-off by the quantum backreaction. A new time scale emerges
that separates the linear from the non-linear regimes. This scale is non-
perturbative in the coupling and initial amplitude. The non-perturbative
evolution is studied in the large the N limit for the O(N) vector model. A
combination of numerical analysis and the multitime scale analysis reveals the
presence of unstable bands in the nonlinear regime. These are associated with
power law growth of quantum fluctuations, that result in power law relaxation
and dissipation with non-universal and non-perturbative dynamical anomalous
exponents.We find substantial particle production during this non-linear evolu-
tion which is of the same order as that in the linear regime. The expectation
value of the scalar field vanishes asymptotically transferring all of the
initial energy into produced particles via the non-linear resonances in the
unbroken symmetry phase.The effective mass for the quantum modes tends asympto-
tically to a constant plus oscillating O(1/t) terms. This slow decay causes the
power behaviour in the modes which become free for t=infty. We derive a simple
expression for the equation of state for the fluid of produced particles that
interpolates between radiation-type and dust-type equations according to the
initial value of the order parameter for unbroken symmetry. For broken symmetry
the produced particles are massless Goldstone bosons with a radiation-type
equation of state. We find the onset of a novel form of dynamical Bose
condensation in the collisionless regime in the absence of thermalization.
