Physics – High Energy Physics – High Energy Physics - Phenomenology
Scientific paper
2001-05-29
Nucl.Phys.A699:847-886,2002
Physics
High Energy Physics
High Energy Physics - Phenomenology
48 pages, 20 figures, Nucl.Phys.A version, minor wording changes, reference added
Scientific paper
10.1016/S0375-9474(01)01295-7
We compute the nonequilibrium real-time evolution of an O(N)-symmetric scalar quantum field theory from a systematic 1/N expansion of the 2PI effective action to next-to-leading order, which includes scattering and memory effects. In contrast to the standard 1/N expansion of the 1PI effective action, the next-to-leading order expansion in presence of a possible expectation value for the composite operator leads to a bounded time evolution where the truncation error may be controlled by higher powers in 1/N. We present a detailed comparison with the leading-order results and determine the range of validity of standard mean field type approximations. We investigate ``quench'' and ``tsunami'' initial conditions frequently used to mimic idealized far-from-equilibrium pion dynamics in the context of heavy-ion collisions. For spatially homogeneous initial conditions we find three generic regimes, characterized by an early-time exponential damping, a parametrically slow (power-law) behavior at intermediate times, and a late-time exponential approach to thermal equilibrium. The different time scales are obtained from a numerical solution of the time-reversal invariant equations in 1+1 dimensions without further approximations. We discuss in detail the out-of-equilibrium behavior of the nontrivial n-point correlation functions as well as the evolution of a particle number distribution and inverse slope parameter.
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