Abstract
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 model in the energy range from E_mathrm {lab}=1.23 AGeV to sqrt{s_mathrm {NN}}=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 several decay and re-formation chains involving resonances and taking into account inelastic, pseudo-elastic and string excitation 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 microscopic dynamical simulation, that the chemical freeze-out at all energies coincides with langle Erangle /langle Nrangle approx 1 GeV, while other criteria, like s/T^3=7 and n_mathrm {B}+n_{bar{mathrm {B}}}approx 0.12 fm^{-3} are limited to higher collision energies.
Highlights
The collision of heavy ions in today’s largest particle accelerators provides an excellent tool to explore nuclear and subnuclear matter under extreme conditions as they occur e.g. in neutron stars, around black holes or in the early universe
We focus on the chemical freeze-out of pions which decouple in the volume with |z| ≤ 5 fm
In this article we have developed a novel approach to determine the chemical freeze-out hyper-surface directly from a microscopic simulation
Summary
The collision of heavy ions in today’s largest particle accelerators provides an excellent tool to explore nuclear and subnuclear matter under extreme conditions as they occur e.g. in neutron stars, around black holes or in the early universe. At the chemical freeze-out inelastic flavour changing reactions cease (particle yields get fixed) and at the kinetic freeze-out elastic reactions cease (particle 4-momenta get fixed) and the system decouples This behavior is reflected in the scattering rates as shown in [8] where the chemical freeze-out is estimated to occur at τchem = 6 fm. The Delta will be further tracked backwards and if it was formed by a pion, the procedure continues further backwards in time with the newly found pion until the first creation of the pion These space-time points define the hyper-surface of the chemical freeze-out of the pions. A transition to a deconfined stage is not explicitly included in the cascade mode employed here
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