Simulation And Research Of Centrality Depedence Of Freeze-out Parameters From RHIC Beam Energy Scan

Because of the idea of asymptotic freedom, in the region of high temperaturedense nuclear matter produced by the high energy heavy-ion collision, quarks andgluons deconfine from the hadrons, and then quark gluon plasma is formed. Thereare two kinds of phase transitions between this new phase of matter and the normalhadronic phase of matter: crossover and first order phase transition. By employingtemperature and baryon chemical potential as axises, one can describe the phasetransition between quark gluon plasma and hadronic matter within a phase diagram.Because high collision energy corresponds low baryon chemical potential, bychanging the collision energy, beam energy scan program can control the positionwhere the transition between quark-gluon plasma and hadronic matter happened,then explore different phase transition points at the phase diagram, Generally,people think that the critical point which connect the two kinds of phase transitionexists within the energy range of RHIC beam energy scan. In order to reach theinformation of the phase transition, the analysis of chemical freeze-out and kineticfreeze-out of the source produced by the collision is an accessible way. Sincetheories are incomplete, people usually understand experimental results mainlythrough various models. In order to further understand the centrality dependence offreeze-out parameters at RHIC beam energy scan, this thesis uses default AMPTmodel to simulate Au+Au collisions at the energy range of RHIC beam energy scan.After fitting synthesis transverse momentum with Boltzmann distribution, inversecoefficient is derived. Then by subtracting the part relating to collective movementfrom the inverse coefficient, the kinetic temperature is obtained. By observing thevariations of thermal movement and collective movement with the evolution of thesource, this thesis evaluates the relationship between the centrality dependence ofkinetic freeze-out parameters and the time when kinetic freeze-out happens. Byobserving the change of the yield of hadrons with the evolution of the source, wedetermine the time when chemical freeze-out happens, and then obtain the ratios ofdifferent kinds of hadrons. Comparing the chemical freeze-out parameters derivedfrom fitting the ratios of the yields of hadrons at chemical freeze-out and finalhadronic state with THERMUS model, we analyze the impact of the slight variationof hadron yields happens after the chemical freeze-out on the collisions withdifferent centralities and collision energies. The difference between centralitydependence of chemical freeze-out parameters with different collision energies ismore obvious when calculated through the ratios at chemical freeze-out. After theanalysis of centrality dependence of kinetic freeze-out parameters and chemical freeze-out parameters under the collision energy of10,15,20and30GeV, this paperfinds out that when the collision energy is10GeV, the centrality dependence ofkinetic freeze-out parameters and chemical freeze-out parameters is very differentwith the situation under other collision energies.