QCD临界终点与重子数扰动

QCD critical end point and baryon number fluctuation

相对论性重离子对撞的目的之一是寻找量子色动力学(Quantum Chromodynamics,QCD)临界终点(Critical End Point,CEP),实验中测到的净质子数扰动呈现出非单调的行为,这暗示了CEP的存在。本文使用了3味PNJL(Polyakov-loop Nambu-Jona-Lasinio)模型,沿着从实验数据中拟合的化学冻结线,计算了重子数扰动累积量之比C4/C2随对撞能量的变化。结果发现,基于平衡态假设,在对撞能量7.7~200 GeV范围内,随着对撞能量的降低,C4/C2先缓慢下降后上升,这与实验数据一致。这也暗示平衡态假设可以用于探索重离子对撞后系统的演化行为,揭示了相变线跟化学冻结线的关系对观测量的影响非常重要。

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.