The ratio of viscosity to entropy density in a pion gas satisfies the KSS holographic bound

Abstract

We evaluate the ratio of shear viscosity to entropy density in a pion gas employing the Uehling-Uehlenbeck equation and experimental phase-shifts parameterized by means of the SU(2) Inverse Amplitude Method. We find that the ratio for this monocomponent gas stays well above the KSS 1/(4 pi) bound. We find similar results with other sets of phase shifts and conclude the bound is nowhere violated.

Comments: 2 page text, three figures. V2: short comment and graph added to assert that a minimum of eta/s is not discarded from the hadron, low T side in a heavy-ion collision