Holographic Viscosity of Fundamental Matter

TL;DR

This paper uses holographic duality to analyze how fundamental matter influences the viscosity in finite-temperature gauge theories, showing that the viscosity to entropy ratio saturates the universal bound and is enhanced at strong coupling.

Contribution

It demonstrates that including backreacted fundamental matter in holographic models leads to saturation of the viscosity bound and enhances transport coefficients at strong coupling.

Findings

Viscosity to entropy ratio saturates the universal bound.

Fundamental matter contribution to viscosity is enhanced at strong coupling.

Transport coefficients are similarly enhanced with fundamental matter.

Abstract

A holographic dual of a finite-temperature SU(N_c) gauge theory with a small number of flavours N_f << N_c typically contains D-branes in a black hole background. By considering the backreaction of the branes, we demonstrate that, to leading order in N_f/N_c, the viscosity to entropy ratio in these theories saturates the conjectured universal bound eta/s >= 1/4\pi. The contribution of the fundamental matter eta_fund is therefore enhanced at strong 't Hooft coupling lambda; for example, eta_fund ~ lambda N_c N_f T^3 in four dimensions. Other transport coefficients are analogously enhanced. These results hold with or without a baryon number chemical potential.