The Effect of Light Nuclei on Chemical Freeze-out Parameters at RHIC Energies

This study investigates the chemical freeze-out of hadrons, including light-flavor, strange-flavor particles, and light nuclei, produced in Au+Au collisions at the Relativistic Heavy Ion Collider (RHIC). Utilizing the Thermal-FIST thermodynamic statistical model, we analyzed various particle sets: those inclusive of light nuclei, those exclusive of light nuclei, and those solely comprising light nuclei. We have determined the chemical freeze-out parameters at $\sqrt{s_\text{NN}}=$7.7-200 GeV and four centralities. A significant finding was a decrease in the chemical freeze-out temperature $T_{\textrm{ch}}$ upon the inclusion of light nuclei, with an even more pronounced reduction when considering light nuclei yields exclusively. This indicates that light nuclei formation occurs at a later stage in the system's evolution at RHIC energies. We present parameterized formulas that describe the energy dependence of the chemical freeze-out temperature $T_{\textrm{ch}}$ and baryon chemical potential $\mu_B$ for three distinct particle sets in central Au+Au collisions at RHIC energies. Our results reveal the presence of at least three distinct freeze-out hyper-surfaces at RHIC energies, corresponding to different chemical freeze-out temperatures: a light flavor freeze-out temperature of $T_L$=150.2$\pm$6 MeV, a strange flavor freeze-out temperature $T_s$=165.1$\pm$2.7 MeV, and a light nuclei freeze-out temperature $T_{\textrm{ln}}$=141.7$\pm$1.4 MeV. Notably, at the Large Hadron Collider (LHC) energies, the expected lower freeze-out temperature for light nuclei was not observed. Instead, the chemical freeze-out temperature for light nuclei was found to be approximately 10 MeV higher than that for light flavor hadrons. This discrepancy may suggest different production mechanisms for light nuclei between RHIC and LHC energies.
Comment: 9 pages, 6 figures