Study On Collectivity,Correlation,and Fluctuation In Relativistic Heavy Ion Collisions

Relativistic heavy ion collisions are an effective way for exploring the proper-ties of strongly interacting matter under extreme conditions.The Relativistic Heavy Ion Collider(RHIC)makes two nuclei collide at nearly the speed of light to produce high-temperature and high-density matter,simulating the formation of the quark gluon plasma in the early stage of the Big Bang.In order to study relativistic heavy ion col-lisions,people have developed many phenomenological models that can also describe the non-equilibrium multibody dynamic process in different relativistic heavy ion col-lision systems and at different collision energies.This Ph.D thesis studies the nature of anisotropic flows,HBT correlations,and centrality fluctuations using phenomenologi-cal models and in experimental data analysis.Collectivity,correlation,and fluctuation are sensitive to the initial state of heavy ion collisions and the evolution of the system,providing relevant information of the quark gluon plasmaThe research on anisotropic flows mainly includes the scaling of anisotropic flows,the non-flow effect in multiparticle azimuthal correlations in small collision systems,and flow fluctuations in different collision systems in STAR experiments.We use the AMPT model,the blast wave model,and the coalescence model to deeply study the scaling of anisotropic flows.It is found that the final hadrons satisfying the modified NCQ scaling as a result of the hadronic interaction or the hydrodynamic freeze-out rather than the quark coalescence mechanism.In addition,the flow scaling ratio is sen-sitive to the partonic interaction.The extraction of the anisotropic flow signal is affected by the significant non-flow effect.We calculate the multiparticle correlation using the subevent method,which can effectively reduce the nonflow contribution.The three-particle azimuthal correlation in p+p and p+Pb collisions is calculated using the stan-dard and subevent cumulant methods with the simple models PYTHIA8 and HIJING,which have no collectivity and the results are comparing with the ATLAS experimental data.We predicted the nonflow effect on the asymmetric cumulantac2,3|5{3},which serves as a valuable reference for the future measurement.The anisotropic flow is af-fected by event-by-event fluctuations.We analyzed the flow fluctuation using STAR data in Au+Au and U+U collision systems.It was found that v2{4} has different be-haviors in ultra central collisions due to the different initial geometry fluctuations be-tween Au+Au and U+U collisions.The significant transverse momentum dependence of the flow ratio v2 {4}/v2 {2} and v2 {6}/v2 {4} indicates that flow fluctuations may arise directly in the momentum space through the initial-state correlation or the final-state interaction.The calculation of the normalized cumulant shows that the flow fluctuation is a non-Gaussian fluctuationHBT correlation can be used to effectively extract the information about the size and evolution of the early sources in relativistic heavy ion collisions.The RHIC-BES and FAIR-CBM programs carry out lower-energy collisions where the hadronization and the hadronic evolution are more important,and this is a hotspot in current interna-tional research.Knowledge of the hadronic evolution and the mean-field potential plays an important role in studying collision kinetics and extracting the QCD phase diagram information at RHIC-BES and FAIR-CBM energies.We extended the AMPT model to study the effect of hadronic mean-field potentials on HBT correlations.Pion femoto-scopy shows that the hadronic mean-field potential associated with the equation of state of the medium can affect the evolution of the entire system,the system size of kinetic freeze-out as well as the emission time of particles.On the other hand,the HBT cor-relations for protons,kaons,and antiprotons can be useful in extracting information of their mean-field potentials in the baryon-rich hadronic matter as well as understanding baryon-antibaryon annihilationsThe number and distribution of sources have a significant fluctuation in their trans-verse plane along the pseudorapidity.It is important to study the correlations and fluc-tuations in the pseudorapidity intervals to understand the basic QCD partonic dynamics in strongly coupled non-perturbative area.Centrality fluctuation may affect the fluctu-ations of all physical quantities that have centrality dependence.Based on the Glauber model and the negative binomial distribution,the centrality fluctuation mechanism was studied.It was found that the centrality fluctuations can arise from the multiplicity smearing effect and the centrality decorrelation effect.Studies on the system-size de-pendence of the centrality fluctuation indicates that the smaller collision system or the higher asymmetry of the forward-going and backward-going participants leads to the more obvious centrality fluctuation.This study is helpful in understanding the mecha-nism of particle productions as well as the nature and fluctuation of the source.At the same time,it plays a guiding role in future experimental research on the quantitative analysis of the centrality fluctuations.