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Description
The hadronic transport simulation UrQMD is used to pin down the chemical freeze-out space-time point of each final state pion. In combination with a coarse-grained collision evolution, the local temperature $T$ and the local baryo-chemical potential $\mu_{\rm B}$ are estimated up to the top RHIC energies. The results match thermal model estimates of experimental data with high accuracy. However, here the chemical freeze-out is linked to the space and time dependent relation between the expansion and the scattering rate and not to the onset of deconfinement. Moreover, typical freeze-out criteria proposed in the literature are investigated on the reconstructed chemical freeze-out hyper-surface. The average energy per particle criterion $\langle E\rangle/\langle N\rangle=1$ is confirmed within the whole energy range with deviations up to 20% depending on the meson/baryon content of the colliding system. The entropy per baryon criterion $s/T^3=7$ and the total baryon density criterion $n_{\rm B}+n_{\bar{\rm B}}=0.12$ fm$^{-3}$ are confirmed above 7 and 20 GeV, respectively. Finally, the combined UrQMD + coarse-graining model is used to constrain the space and time dependence of the shear viscosity to entropy density ratio $\eta/s$ in central Au+Au collisions at a beam energy of 1.23 AGeV. The results indicate an intricate sensitivity to the different stages of the collision at this energy, i.e. the compression phase and expansion phase.
