The Shear Viscosity to Entropy Density Ratio of Hagedorn States

TL;DR

This paper calculates the shear viscosity to entropy density ratio of Hagedorn states using a transport model, revealing it approaches or drops below the theoretical bound near the Hagedorn temperature.

Contribution

It provides the first calculation of the shear viscosity to entropy density ratio for Hagedorn states using the Green-Kubo method and a recent mass spectrum.

Findings

Ratio approaches or falls below 1/4π near T_H

Shear viscosity decreases significantly near the Hagedorn temperature

Results suggest Hagedorn states may exhibit nearly perfect fluid behavior

Abstract

The fireball concept of Rolf Hagedorn, developed in the 1960's, is an alternative description of hadronic matter. Using a recently derived mass spectrum, we use the transport model GiBUU to calculate the shear viscosity of a gas of such Hagedorn states, applying the Green-Kubo method to Monte-Carlo calculations. Since the entropy density is rising ad infinitum near $T_H$, this leads to a very low shear viscosity to entropy density ratio near $T_H$. Further, by comparing our results with analytic expressions, we find a nice extrapolation behavior, indicating that a gas of Hagedorn states comes close or even below the boundary $1/4\pi$ from AdS-CFT.