Ratio of bulk to shear viscosity in a quasigluon plasma: from weak to strong coupling

TL;DR

This paper investigates how the ratio of bulk to shear viscosity in a gluon plasma varies with temperature, showing a transition from weak to strong coupling behavior influenced by the medium's conformality.

Contribution

It introduces an effective kinetic theory approach to analyze viscosity ratios, capturing both weak and strong coupling behaviors within a unified framework.

Findings

Viscosity ratio depends quadratically on conformality in weak coupling.

Linear dependence of viscosity ratio observed in strong coupling regimes.

Transition point aligns with the maximum in the scaled interaction measure.

Abstract

The ratio of bulk to shear viscosity is expected to exhibit a different behaviour in weakly and in strongly coupled systems. This can be expressed by its dependence on the squared sound velocity. In the high temperature QCD plasma at small running coupling, the viscosity ratio is uniquely determined by a quadratic dependence on the conformality measure, whereas in certain strongly coupled and nearly conformal theories this dependence is linear. Employing an effective kinetic theory of quasiparticle excitations with medium-modified dispersion relation, we analyze the ratio of bulk to shear viscosity of the gluon plasma. We show that in this approach, depending on the temperature, the viscosity ratio exhibits either of these dependencies found by means of weak coupling perturbative or strong coupling holographic techniques. The turning point between the two different dependencies is located around the maximum in the scaled interaction measure.