Low-shear QCD plasma from perturbation theory

TL;DR

This paper demonstrates that the low shear viscosity to entropy density ratio of quark-gluon plasma near the deconfinement temperature can be explained using perturbative QCD, challenging the view that it is inherently non-perturbative.

Contribution

The authors show that perturbative QCD, with proper resummation and scale setting, can account for the low shear viscosity to entropy density ratio observed in quark-gluon plasma.

Findings

Perturbative QCD can explain $/$ near $T_c$

Resummation and scale setting are crucial for accurate predictions

Temperature dependence of $/$ aligns with hydrodynamic constraints

Abstract

We argue that the phenomenologically inferred ratio of shear viscosity to entropy density of the quark-gluon plasma, $\eta/\s < 0.5$ near the deconfinement temperature $T_c$, can be understood from perturbative QCD. To rebut the widespread, opposite view we first show that, and why, the existing leading order result in (fixed) coupling should not be further expanded in logarithms. Emphasizing then that the resummation mandatory for screening also settles the often neglected question of scale setting for the running coupling, we establish a temperature dependence of $\eta/\s$ which agrees well with constraints from hydrodynamics.