Beyond eta/s = 1/4pi

TL;DR

The paper investigates violations of the Kovtun–Son–Starinets viscosity bound in four-dimensional superconformal field theories with $c\neq a$ by employing a low-energy five-dimensional gravity dual with controlled curvature corrections. It shows that the bound can be violated when the central charges satisfy $c\sim a\gg1$ and $\delta=(c-a)/c>0$, with the leading correction to $\eta/s$ tied to $\alpha_3\sim\delta/8$ in the effective action; however, a reliable gravity dual generally requires $\delta\ll1$, limiting where violations can be demonstrated. The authors compute $\delta$ across various ${\cal N}=1$ and ${\cal N}=2$ SCFTs, including exactly marginal couplings and isolated fixed points, and identify string-theory realizations (e.g., F-theory constructions) that yield small $\delta$ and hence controllable dual descriptions. They apply the framework to model the sQGP, deriving holographic predictions for $\varepsilon/\varepsilon_0$ and $\eta/s$ and discussing tensions with lattice QCD results, which motivate extending the universality class to include $R^4$ corrections and other effects. The work clarifies the conditions under which KSS-bounded violations occur in consistent holographic setups and discusses the implications and limitations for connecting holography to real-world strongly coupled plasmas.

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.