Pseudothermalization of the quark-gluon plasma

TL;DR

This paper reviews recent kinetic theory work showing that quark-gluon plasma undergoing Bjorken expansion exhibits a pseudothermal attractor in all moments of the distribution function, applicable even far from equilibrium, with implications for heavy-ion collision modeling.

Contribution

It demonstrates the existence of a pseudothermal attractor in both relaxation time approximation and QCD kinetic theory, extending to early times far from equilibrium.

Findings

Existence of a pseudothermal attractor in RTA and QCD kinetic theory.

Attractor extends to early, far-from-equilibrium times.

Improved freeze-out prescriptions like anisotropic hydrodynamics perform better in high-energy collisions.

Abstract

In this proceedings contribution I review recent work in kinetic theory which demonstrates that, for system undergoing Bjorken expansion, there exists an attractor in all moments of the one-particle distribution function. I discuss how this attractor emerges in both exact solutions obtained in relaxation time approximation (RTA) and the effective kinetic theory approach to high-temperature quantum chromodynamics (QCD). The QCD effective kinetic theory collisional kernel used includes both elastic (2 <-> 2) and LPM-resummed inelastic (2 <-> 1) contributions. The results obtained indicate that a pseudothermal attractor exists in both RTA and QCD kinetic theory and that their respective attractors can be extended to early times when the system is far from equilibrium. Finally, I discuss how knowledge of the QCD effective kinetic theory attractor can be used to assess different hydrodynamic freeze-out prescriptions used in heavy-ion phenomenology. The results obtained show that improved freeze-out prescriptions such as anisotropic hydrodynamics perform better in conditions corresponding to those generated in high-multiplicity pA and pp collisions, e.g. short lifetime and high inverse Reynolds number.