Probing chemical freeze-out criteria in relativistic nuclear collisions with coarse grained transport simulations
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Regular Article - Theoretical Physics
Probing chemical freeze-out criteria in relativistic nuclear collisions with coarse grained transport simulations Tom Reichert1,2,a , Gabriele Inghirami3,4 , Marcus Bleicher1,2,5,6 1
Institut für Theoretische Physik, Goethe Universität Frankfurt, Max-von-Laue-Str. 1, 60438 Frankfurt am Main, Germany Helmholtz Research Academy Hesse for FAIR, Campus Frankfurt, Max-von-Laue-Str. 12, 60438 Frankfurt, Germany 3 Department of Physics, University of Jyväskylä, PO Box 35, 40014 Jyväskylä, Finland 4 Helsinki Institute of Physics, University of Helsinki, PO Box 64, 00014 Helsinki, Finland 5 GSI Helmholtzzentrum für Schwerionenforschung GmbH, Planckstr. 1, 64291 Darmstadt, Germany 6 John von Neumann-Institut für Computing, Forschungszentrum Jülich, 52425 Jülich, Germany
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Received: 15 July 2020 / Accepted: 5 October 2020 © The Author(s) 2020 Communicated by Laura Tolos
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 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 freezeout 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 E/N ≈ 1 GeV, while other criteria, like s/T 3 = 7 and n B + n B¯ ≈ 0.12 fm−3 are limited to higher collision energies.
1 Introduction 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. Matter created under these conditions sustains tremendous temperatures, pressures and densities in volumes on the order of ∼ 1000 fm3 over timescales of 10−23 s. Since separating quarks creates new quark-anti-quark pairs from the vacuum in order to bind to color neutral a e-mail:
hadrons, a direct measurement of the inner degrees of freedom of strongly interacting matter in heavy ion collisions is, unfortunately, still impossible. In contrast to QED, this phenomenon called confinement forbids perturbative calculations at small momenta. Access to the early and intermediate stage of a collision can be gained via electromagnetic probes e.g. with real photons and virtual photons in the dilepton channel [1–5], via the study of hadrons produced at the chemical freeze-out [6,7], or by flow observables, like v1 , v2 , v3 , . . .. Typically, the final state of a heavy
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