# $\eta/s$ of a Relativistic Hadron Gas at RHIC: Approaching the AdS/CFT bound?

## TL;DR

This paper investigates the shear-viscosity to entropy-density ratio of a hadron gas at RHIC, finding it increases with temperature and challenges hydrodynamic models, implying the low viscosity originates in the partonic phase.

## Contribution

It provides a calculation of /s in the hadron gas phase both in and out of chemical equilibrium, highlighting its implications for understanding the low-viscosity quark-gluon plasma.

## Key findings

- /s increases with temperature in the hadron gas phase.
- High /s values challenge the applicability of viscous hydrodynamics.
- Low viscosity at RHIC likely originates in the partonic phase.

## Abstract

Ultrarelativistic heavy-ion collisions at the Relativistic Heavy Ion Collider (RHIC) are thought to have produced a state of matter called the quark-gluon plasma, characterized by a very small shear-viscosity to entropy-density ratio $\eta/s$, near the lower bound predicted for that quantity by AdS/CFT methods. As the produced matter expands and cools, it evolves through a phase described by a hadron gas with rapidly increasing $\eta/s$. We calculate $\eta/s$ as a function of temperature in this phase both in and out of chemical equilibrium and find that its value poses a challenge for viscous relativistic hydrodynamics, which requires small values of $\eta/s$ in order to successfully describe the collective flow observables at RHIC. We therefore conclude that the origin of the low viscosity matter at the RHIC must be in the partonic phase of the reaction.