Early-Stage Shear Viscosity far from Equilibrium via Holography

TL;DR

This paper uses holography to study how shear viscosity of quark-gluon plasma behaves far from equilibrium, revealing significant variations during early collision phases that impact experimental data interpretation.

Contribution

It extends shear viscosity analysis to far-from-equilibrium conditions using holography, revealing dynamic changes in the viscosity-to-entropy ratio during initial QGP formation.

Findings

Shear viscosity to entropy density ratio decreases below 60% during initial heating.

The ratio overshoots to 110% of the near-equilibrium value.

Results are relevant for interpreting flow measurements at RHIC and LHC.

Abstract

Shear viscosity is a crucial property of QCD matter which determines the collective behavior of the the quark-gluon plasma (QGP) in ultrarelativistic heavy-ion collisions. Extending the near-equilibrium, high-precision investigations in theory and experiment, we take into account the fact that, in a collision, the QGP is generated far from equilibrium. We use the AdS/CFT correspondence to study a strongly coupled plasma and find a significant impact on the ratio of shear viscosity to entropy density, $\eta/s$. In particular, we investigate the initial heating phase and find a decrease reaching down to below 60% followed by an overshoot to 110% of the near-equilibrium value. This finding might be highly relevant for the extraction of transport coefficients from anisotropic flow measurements at RHIC and LHC.