Energy and centrality dependence of chemical freeze-out thermodynamics parameters

Abstract

Driven by the Beam Energy Scan (BES) program at the RHIC, researches and discussions on the QCD phase diagram have flourished recently. In order to provide a reference from microscopic transport models, we performed a systematic analysis, using a multiphase transport (AMPT) model for the particle yields and a statistical model (THERMUS) for the thermal fit, for Au+Au collisions at $\sqrt{s_{\text{NN}}}$=7.7-200 GeV. It is found that at a fixed collision centrality the chemical freeze-out parameter, temperature $T_{\text{ch}}$, increases with collision energy and somehow saturates at certain values of $T_{\text{ch}}$ in collisions near $\sqrt{s_{\text{NN}}}$=10 GeV, indicating the limiting temperature in hadronic interactions; meanwhile the baryon chemical potential $\mu_B$ decrease with the collision energy. The saturation temperature is also found to be dependent on partonic interaction. At a given collision energy, it is found that both $T_{\text{ch}}$ and $\mu_B$ decrease towards more peripheral collisions in the grand canonical approach. The energy and centrality dependence of other chemical freeze-out parameters, strangeness chemical potential $\mu_S$, strangeness undersaturation factor $\gamma_S$, and the volume of the fireball $V$ are also presented in this paper. The chemical potential ratio $\mu_s/\mu_B$ is also compared with lattice QCD calculation. The AMPT default model gives better descriptions on both the particle yields and the chemical freeze-out parameters than those from the AMPT string-melting model.