On the violation of the holographic viscosity versus entropy KSS bound in non-relativistic systems

Abstract

A computation of the quotient of shear viscosity and entropy density, or the Kovtun, Son and Starinets (KSS) number η/s, is performed in the non-relativistic and classical regime, first in chiral perturbation theory, and then in the SO(g+1)/SO(g) non-linear sigma model in the large g limit. Both are field theories stemming from a renormalizable sigma model, but, in spite of that, we explicitly calculate how one avoids the KSS bound by increasing the number of degenerate pions sufficiently. However, we argue that particle production could still keep the validity of the KSS bound in the weak sense. We also show how a large number of molecular isomers (which we estimate in terms of simple molecular properties) could avoid the bound in the strong sense. This might be possible with carbon based molecules. We finally argue that a measurement of η/s in heavy-ion collisions might be turned into an upper bound on the number of hadron resonances.