A first estimate of η/s in Au+Au reactions at Elab = 1.23 AGeV

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 lab = 1.23 A GeV (or s NN = 2.4 GeV ). This data allows to estimate the shear viscosity over entropy density ratio, η / 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 η / s ≈ 0.65 ± 0.15 (or ( 8 ± 2 ) ( 4 π ) − 1 ) at such low energies.