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

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 $E_{\mathrm{lab}}=1.23$~$A$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 $\eta /s\approx 0.65\pm 0.15$ (or $(8\pm 2)(4\pi {)}^{-1}$) at such low energies.

Comments: 6 pages, 5 figures