Matching the Nonequilibrium Initial Stage of Heavy Ion Collisions to Hydrodynamics with QCD Kinetic Theory

TL;DR

This paper develops a practical framework using QCD kinetic theory and nonequilibrium Green's functions to connect the early nonequilibrium stage of heavy ion collisions to hydrodynamic evolution, improving constraints on initial conditions.

Contribution

It introduces a novel method to bridge classical field simulations and hydrodynamics in heavy ion collisions using Green's functions from QCD kinetic theory.

Findings

Framework can be integrated into existing hydrodynamic models.

Provides bounds on the applicability of hydrodynamics based on Green's functions.

Enhances constraints on early energy density and transport properties.

Abstract

High-energy nuclear collisions produce a nonequilibrium plasma of quarks and gluons which thermalizes and exhibits hydrodynamic flow. There are currently no practical frameworks to connect the early particle production in classical field simulations to the subsequent hydrodynamic evolution. We build such a framework using nonequilibrium Green's functions, calculated in QCD kinetic theory, to propagate the initial energy-momentum tensor to the hydrodynamic phase. We demonstrate that this approach can be easily incorporated into existing hydrodynamic simulations, leading to stronger constraints on the energy density at early times and the transport properties of the QCD medium. Based on (conformal) scaling properties of the Green's functions, we further obtain pragmatic bounds for the applicability of hydrodynamics in nuclear collisions.