Abstract
Non-equilibrium Green's functions provide an efficient way to describe the evolution of the energy-momentum tensor during the early time pre-equilibrium stage of high-energy heavy ion collisions. Besides their practical relevance they also provide a meaningful way to address the question when and to what extent a hydrodynamic description of the system becomes applicable. Within the kinetic theory framework we derive a new method to calculate time dependent non-equilibrium Green's functions describing the evolution of energy and momentum perturbations on top of an evolving far-from-equilibrium background. We discuss the approach towards viscous hydrodynamics along with the emergence of various scaling phenomena for conformal systems. By comparing our results obtained in the relaxation time approximation to previous calculations in Yang-Mills kinetic theory, we further address the question which macroscopic features of the energy momentum tensor are sensitive to the underlying microscopic dynamics.
Highlights
Ultrarelativistic heavy-ion experiments carried out at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC) have produced a new state of matter where quarks and gluons are liberated from the incoming nuclei [1,2,3,4]
By comparing our results obtained in the relaxation time approximation to previous calculations in Yang-Mills kinetic theory, we further address the question which macroscopic features of the energy-momentum tensor are sensitive to the underlying microscopic dynamics
Due to the simple structure of the relaxation time approximation considered in this work, we obtained a closed set of moment equations for the evolution of the dimension-four moments Cml and δCml, which are relevant to study the evolution of the energy-momentum tensor, that is determined by the lowest order (l ≤ 2) moments
Summary
Ultrarelativistic heavy-ion experiments carried out at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC) have produced a new state of matter where quarks and gluons are liberated from the incoming nuclei [1,2,3,4]. Based on an underlying microscopic description in QCD kinetic theory, the initial conditions for the energy-momentum tensor TμνðτhydroÞ at the beginning of the hydrodynamic phase are hereby obtained from the initial energy-momentum tensor Tμνðτ0Þ, by determining the evolution of macroscopic quantities, based on nonequilibrium Green’s functions of the energy-momentum tensor [48,49]. This new framework, dubbed as KøMPøST [48,49], has proven to be a powerful tool to describe the preequilibrium evolution of heavy-ion collisions on an event-by-event basis [51,52]. Some technical aspects of our work are briefly discussed in the Appendix
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