Nonequilibrium QCD: Interplay of hard and soft dynamics in high-energy multigluon beams

Abstract

A quantum-kinetic formulation of the dynamical evolution of a high-energy nonequilibrium gluon system at finite density is developed to study the interplay between quantum fluctuations of high-momentum (hard) gluons and the low-momentum (soft) mean color field that is induced by the collective motion of the hard particles. From the exact field equations of motion of QCD, a self-consistent set of approximate quantum-kinetic equations are derived by separating hard and soft dynamics and choosing a convenient axial-type gauge. This set of master equations describes the momentum space evolution of the individual hard quanta, the space-time development of the ensemble of hard gluons, and the generation of the soft mean field by the current of the hard particles. The quantum-kinetic equations are approximately solved to order ${g}^{2}(1+g\ifmmode \bar{A}\else \={A}\fi{})$ for a specific example, namely, the scenario of a high-energy gluon beam along the light cone, demonstrating the practical applicability of the approach.