Beyond eta/s = 1/4pi

TL;DR

This paper demonstrates that the Kovtun-Soninets viscosity bound can be violated in superconformal gauge theories with unequal central charges, providing new examples and discussing implications for QCD plasma.

Contribution

It introduces new examples of gauge theories where the viscosity bound is violated, challenging previous assumptions of universality.

Findings

Viscosity bound is violated in certain superconformal theories

New string theory and gauge theory examples of bound violation

Implications for understanding QCD plasma viscosity

Abstract

We use low-energy effective description of gauge theory/string theory duality to argue that the Kovtun-Son-Starinets viscosity bound is generically violated in superconformal gauge theories with non-equal central charges $c\ne a$. We present new examples (of string theory constructions and of gauge theories) where the bound is violated in a controllable setting. We consider the comparison of results from AdS/CFT calculations to the QCD plasma in the context of this discussion.