The viscosity bound in string theory

TL;DR

This paper investigates the viscosity bound in string theory models, providing evidence that quantum corrections generally uphold the bound, but matter fields can cause violations, and discusses extending AdS/CFT to RHIC physics.

Contribution

It analyzes string theory models to assess the validity of the viscosity bound and explores the effects of matter fields and quantum corrections on this conjecture.

Findings

Quantum corrections typically keep η/s above the bound

Matter fields can violate the viscosity bound

Proposes extending AdS/CFT methods to RHIC phenomenology

Abstract

The ratio of shear viscosity to entropy density $\eta/s$ of any material in nature has been conjectured to have a lower bound of $1/4\pi$, the famous KSS bound. We examine string theory models for evidence in favour of and against this conjecture. We show that in a broad class of models quantum corrections yield values of $\eta/s$ just above the KSS bound. However, incorporating matter fields in the fundamental representation typically leads to violations of this bound. We also outline a program to extend AdS/CFT methods to RHIC phenomenology.