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We introduce a novel approach based on elas- tic and inelastic scattering rates to extract the hyper-surface of the chemical freeze-out from a hadronic transport model in the energy range from Elab = 1.23 AGeV to √sNN = 62.4 GeV. For this study, the Ultra-relativistic Quantum Molecular Dynamics (UrQMD) model combined with a coarse-graining method is employed. The chemical freeze- out distribution is reconstructed from the pions through sev- eral decay and re-formation chains involving resonances and taking into account inelastic, pseudo-elastic and string excita- tion reactions. The extracted average temperature and baryon chemical potential are then compared to statistical model analysis. Finally we investigate various freeze-out criteria suggested in the literature. We confirm within this micro- scopic dynamical simulation, that the chemical freeze-out at all energies coincides with ⟨E⟩/⟨N⟩ ≈ 1 GeV, while other criteria, like s/T 3 = 7 and nB +nB ̄ ≈ 0.12 fm−3 are limited to higher collision energies.
We introduce a novel approach based on elastic and inelastic scattering rates to extract the hyper-surface of the chemical freeze-out from a hadronic transport simulation. We use the Ultra-relativistic Quantum Molecular Dynamics (UrQMD) model to extract the chemical freeze-out hyper-surface of pions and kaons in the energy range from Elab = 1:23A GeV to √SNN = 7.7 GeV. By employing a coarse-graining procedure, we can extract the local temperature T and baryo-chemical potential μB on the chemical freeze-out surface and compare them to results from statistical model analysis. We find good agreement between the pion chemical freeze-out line extracted from the simulation and the freeze-out line from the statistical model extracted from data. In addition the simulations also hint towards the existence of a flavor hierarchy similar to the one observed in recent lattice QCD calculations.
The HADES experiment at GSI has recently provided data on the flow coefficients v1,..., v4 for protons in Au+Au reactions at Elab = 1.23 AGeV (or √sNN = 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)(4π)−1) at such low energies.