Equilibration of anisotropic quark-gluon plasma produced by decays of color flux tubes

TL;DR

This study models the thermalization of anisotropic quark-gluon plasma from color flux tube decays using kinetic equations, revealing how viscosity influences the approach to hydrodynamics and the persistence of collective oscillations.

Contribution

It introduces a kinetic framework connecting decay rates and viscosity to plasma equilibration, highlighting limitations of viscous hydrodynamics at higher viscosities.

Findings

Lower viscosity (4 pi eta/s = 1) leads to rapid approach to hydrodynamics within 1-2 fm/c.

Higher viscosity (4 pi eta/s ≥ 3) results in persistent plasma oscillations.

The study suggests extending hydrodynamic models to include dissipative phenomena at higher viscosities.

Abstract

A set of kinetic equations is used to study equilibration of the anisotropic quark-gluon plasma produced by decays of color flux tubes possibly created at the very early stages of ultra-relativistic heavy-ion collisions. The decay rates of the initial color fields are given by the Schwinger formula, and the collision terms are treated in the relaxation-time approximation. By connecting the relaxation time with viscosity we are able to study production and thermalization processes in the plasma characterized by different values of the ratio of the shear viscosity to entropy density, eta/s. For the lowest (KSS) value of this ratio, 4 pi eta/s = 1, and realistic initial conditions for the fields, the system approaches the viscous-hydrodynamics regime within 1-2 fm/c. On the other hand, for larger values of the viscosity, 4 pi eta/s greater or equal to 3, the collisions in the plasma become inefficient to destroy collective phenomena which manifest themselves as oscillations of different plasma parameters. The presence of such oscillations brings in differences between the kinetic and hydrodynamic descriptions, which suggest that the viscous-hydrodynamics approach after 1-2 fm/c is not complete if 4 pi eta/s is greater or equal to 3 and should be extended to include dissipative phenomena connected with color conductivity.