A first estimate of $\eta/s$ in Au+Au reactions at E$_{\rm lab}=1.23$ $A$GeV

TL;DR

This paper estimates the shear viscosity to entropy ratio, η/s, in gold-gold collisions at low energy using experimental flow data and transport simulations, providing a first such estimate at these energies.

Contribution

It introduces a novel coarse graining method to estimate η/s from flow data at low energies, which has not been done before.

Findings

Estimated η/s ≈ 0.65 ± 0.15 at low energies

First estimate of η/s in Au+Au reactions at E_lab=1.23 AGeV

Provides a benchmark for low-energy QGP properties

Abstract

The HADES experiment at GSI has recently provided data on the flow coefficients $v_1,...,v_4$ for protons in Au+Au reactions at $E_{\rm lab} = 1.23$~$A$GeV (or $\sqrt{s_\mathrm{NN}}=2.4$ GeV). This data allows to estimate the shear viscosity over entropy ratio, $\eta/s$ at low energies via a coarse graining analysis of the UrQMD transport simulations of the flow harmonics in comparison to the experimental data. By this we can provide for the first time an estimate of $\eta/s\approx0.65\pm0.15$ (or $(8\pm2)\,(4\pi)^{-1}$) at such low energies.