Estimating $\eta/s$ of QCD matter at high baryon densities

TL;DR

This study estimates how the shear viscosity to entropy density ratio of QCD matter varies with collision energy in heavy ion collisions, revealing a decrease from 0.2 to 0.08 as energy increases from 7 to 39 GeV.

Contribution

It applies a combined cascade and viscous hydrodynamics model to RHIC Beam Energy Scan data to estimate the energy dependence of /s in QCD matter.

Findings

/s decreases from 0.2 to 0.08 with increasing energy

Model reproduces experimental data across a range of collision energies

Provides insights into the QCD matter properties at high baryon densities

Abstract

We report on the application of a cascade + viscous hydro + cascade model for heavy ion collisions in the RHIC Beam Energy Scan range, $\sqrt{s_{\rm NN}}=6.3\dots200$ GeV. By constraining model parameters to reproduce the data we find that the effective(average) value of the shear viscosity over entropy density ratio $\eta/s$ decreases from 0.2 to 0.08 when collision energy grows from $\sqrt{s_{\rm NN}}\approx7$ to 39 GeV.