Energy and centrality dependence of chemical freeze-out thermodynamics parameters

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 $\sqrt{{s}_{\mathrm{NN}}}=7.7--200$ GeV. It is found that at a fixed collision centrality the chemical freeze-out parameter, temperature ${T}_{\mathrm{ch}}$, increases with collision energy and somehow saturates at certain values of ${T}_{\mathrm{ch}}$ in collisions near $\sqrt{{s}_{\mathrm{NN}}}=10$ GeV, indicating the limiting temperature in hadronic interactions; meanwhile the baryon chemical potential ${\ensuremath{\mu}}_{B}$ 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 ${T}_{\mathrm{ch}}$ and ${\ensuremath{\mu}}_{B}$ decrease towards more peripheral collisions in the grand canonical approach. The energy and centrality dependence of other chemical freeze-out parameters, strangeness chemical potential ${\ensuremath{\mu}}_{S}$, strangeness undersaturation factor ${\ensuremath{\gamma}}_{S}$, and the volume of the fireball $V$ are also presented in this paper. The chemical potential ratio ${\ensuremath{\mu}}_{s}/{\ensuremath{\mu}}_{B}$ 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.