We compute the nonequilibrium real-time evolution of an O(N)-symmetric scalarquantum field theory from a systematic 1/N expansion of the 2PI effectiveaction to next-to-leading order, which includes scattering and memory effects.In contrast to the standard 1/N expansion of the 1PI effective action, thenext-to-leading order expansion in presence of a possible expectation value forthe composite operator leads to a bounded time evolution where the truncationerror may be controlled by higher powers in 1/N. We present a detailedcomparison with the leading-order results and determine the range of validityof standard mean field type approximations. We investigate ``quench'' and ``tsunami'' initial conditions frequently usedto mimic idealized far-from-equilibrium pion dynamics in the context ofheavy-ion collisions. For spatially homogeneous initial conditions we findthree generic regimes, characterized by an early-time exponential damping, aparametrically slow (power-law) behavior at intermediate times, and a late-timeexponential approach to thermal equilibrium. The different time scales areobtained from a numerical solution of the time-reversal invariant equations in1+1 dimensions without further approximations. We discuss in detail theout-of-equilibrium behavior of the nontrivial n-point correlation functions aswell as the evolution of a particle number distribution and inverse slopeparameter.
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