Transport Properties of QCD at Large $N_c$ and the Gauge/String Duality

TL;DR

This paper demonstrates that in large N_c QCD, the shear and bulk viscosity to entropy density ratios scale with N_c^2 in the confined phase, indicating extremely viscous behavior, and shows gauge/string duality's applicability to such systems.

Contribution

It establishes that holographic models of large N_c QCD predict high viscosity ratios, extending the gauge/string duality's relevance to highly viscous hadronic fluids.

Findings

ta/s pprox; N_c^2 in the confined phase

Gauge/string duality describes highly viscous systems

Results applicable to cold glueball gases

Abstract

Below the deconfinement phase transition large $N_c$ QCD is expected to be a very viscous hadronic fluid because both the shear and bulk viscosity to entropy density ratio, $\eta/s,\zeta/s \sim N_c^2$. In this letter I show that $\eta/s \sim N_c^2$ in the confined phase of holographic models of QCD at large $N_c$ defined in the supergravity approximation. Our results show that the gauge/string duality can be used to describe not only nearly perfect fluids but also extremely viscous systems such as a cold gas of glueballs.