Probing chemical freeze-out criteria in relativistic nuclear collisions with coarse grained transport simulations

Abstract

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 $$_\mathrm {lab}=1.23$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mrow/> <mml:mi>lab</mml:mi> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>1.23</mml:mn> </mml:mrow> </mml:math> AGeV to $$\sqrt{s_\mathrm {NN}}=62.4$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msqrt> <mml:msub> <mml:mi>s</mml:mi> <mml:mi>NN</mml:mi> </mml:msub> </mml:msqrt> <mml:mo>=</mml:mo> <mml:mn>62.4</mml:mn> </mml:mrow> </mml:math> 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 $$\langle E\rangle /\langle N\rangle \approx 1$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mo>⟨</mml:mo> <mml:mi>E</mml:mi> <mml:mo>⟩</mml:mo> <mml:mo>/</mml:mo> <mml:mo>⟨</mml:mo> <mml:mi>N</mml:mi> <mml:mo>⟩</mml:mo> <mml:mo>≈</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:math> GeV, while other criteria, like $$s/T^3=7$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>s</mml:mi> <mml:mo>/</mml:mo> <mml:msup> <mml:mi>T</mml:mi> <mml:mn>3</mml:mn> </mml:msup> <mml:mo>=</mml:mo> <mml:mn>7</mml:mn> </mml:mrow> </mml:math> and $$n_\mathrm {B}+n_{\bar{\mathrm {B}}}\approx 0.12$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi>n</mml:mi> <mml:mi>B</mml:mi> </mml:msub> <mml:mo>+</mml:mo> <mml:msub> <mml:mi>n</mml:mi> <mml:mover> <mml:mrow> <mml:mi>B</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>¯</mml:mo> </mml:mrow> </mml:mover> </mml:msub> <mml:mo>≈</mml:mo> <mml:mn>0.12</mml:mn> </mml:mrow> </mml:math> fm $$^{-3}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mrow/> <mml:mrow> <mml:mo>-</mml:mo> <mml:mn>3</mml:mn> </mml:mrow> </mml:msup> </mml:math> are limited to higher collision energies.

Topics & Concepts