QCD 临界终点与重子数扰动

One of the main goals of relativistic heavy-ion collision (HIC) is to search for the critical end point (CEP) of quantum chromodynamics (QCD), and distribution of the net-proton number from experimental measurements shows non-monotonic behavior, which indicates the existence of a CEP. The purpose of this work is to investigate the relationship between the net-proton number fluctuation and collision energy, and to explain the experimentally measured behavior. This study investigates the three-flavor Polyakov-loop Nambu-Jona-Lasinio (PNJL) model, which contains quark degrees from the NJL (Nambu-Jona-Lasinio) model and effective gluon contributions from Polyakov-loop, based on the equilibrium assumption and mean-field approximation. In addition, we study the phase diagram and C₄/C₂ of baryon number fluctuation as a function of collision energy along the freeze-out lines fitted from experimental data. With an appropriate form of freeze-out line, the collision energy decreases in the region of 7.7~200 GeV, and C₄/C₂ decreases slightly then increases, which is in agreement with the experimental data. Additionally, these results indicate that the equilibrium assumption is appropriate for the exploration of the system evolution after HIC, and the relationship between the freeze-out and phase transition lines is highly sensitive for observables.