Building a testable shear viscosity across the QCD phase diagram

TL;DR

This paper develops a method to calculate the shear viscosity to enthalpy ratio across the QCD phase diagram, aiding the interpretation of heavy-ion collision experiments at varying baryon densities.

Contribution

It introduces a combined HRG and QCD-based model to generate a shear viscosity table across temperature and baryon chemical potential, considering different phase transition scenarios.

Findings

New $ ext{ηT/w}$ values suggest higher initial baryon densities are needed for freeze-out.

The model provides a continuous $ ext{ηT/w}$ map across the phase diagram.

Results impact the understanding of initial conditions in heavy-ion collisions.

Abstract

Current experiments at the Relativistic Heavy Ion Collider (RHIC) are probing finite baryon densities where the shear viscosity to enthalpy ratio $\eta T/w$ of the Quark Gluon Plasma remains unknown. We use the Hadron Resonance Gas (HRG) model with the most up-to-date hadron list to calculate $\eta T/w$ at low temperatures and at finite baryon densities $\rho_B$. We then match $\eta T/w$ to a QCD-based shear viscosity calculation within the deconfined phase to create a table across $\left\{T,\mu_B\right\}$ for different cross-over and critical point scenarios at a specified location. We find that these new $\eta T/w(T,\mu_B)$ values would require initial conditions at significantly larger $\rho_B$, compared to ideal hydrodynamic trajectories, in order to reach the same freeze-out point.