Viscous Hydrodynamics and the Quark Gluon Plasma

TL;DR

This paper reviews how experimental observations of elliptic flow at RHIC, combined with viscous hydrodynamics, suggest that the quark-gluon plasma has an extremely low shear viscosity to entropy ratio, close to the theoretical lower bound.

Contribution

It summarizes the experimental data and theoretical analysis that estimate the shear viscosity of QCD near the phase transition, highlighting its surprisingly small value.

Findings

Shear viscosity to entropy ratio is less than 0.4.

Estimated ratio is approximately (1 to 3) times 1/4π.

Results indicate the QGP behaves as a nearly perfect fluid.

Abstract

One of the most striking results from the Relativistic Heavy Ion Collider is the strong elliptic flow. This review summarizes what is observed and how these results are combined with reasonable theoretical assumptions to estimate the shear viscosity of QCD near the phase transition. A data comparison with viscous hydrodynamics and kinetic theory calculations indicates that the shear viscosity to entropy ratio is surprisingly small, $\eta/s < 0.4$. The preferred range is $\eta/s \simeq (1\leftrightarrow 3) \times 1/4\pi$.