Calculation of shear viscosity using Green-Kubo relations within a parton cascade

TL;DR

This paper calculates the shear viscosity of a gluon gas using Green-Kubo relations and microscopic simulations, revealing the significant impact of bremsstrahlung processes and collision angular distributions on viscosity.

Contribution

It introduces a method to compute shear viscosity from microscopic simulations incorporating pQCD processes, highlighting the effects of bremsstrahlung and collision anisotropy.

Findings

Bremsstrahlung reduces shear viscosity by a factor of 3-8.

Shear viscosity scales as 1/(alpha_s^2 log(1/alpha_s)).

Isotropic collision angles decrease shear viscosity by a factor of 3.

Abstract

The shear viscosity of a gluon gas is calculated using the Green-Kubo relation. Time correlations of the energy-momentum tensor in thermal equilibrium are extracted from microscopic simulations using a parton cascade solving various Boltzmann collision processes. We find that the pQCD based gluon bremsstrahlung described by Gunion-Bertsch processes significantly lowers the shear viscosity by a factor of 3-8 compared to elastic scatterings. The shear viscosity scales with the coupling as 1/(alpha_s^2\log(1/alpha_s)). For a constant coupling constant the shear viscosity to entropy density ratio has no dependence on temperature. Replacing the pQCD-based collision angle distribution of binary scatterings by an isotropic form decreases the shear viscosity by a factor of 3.