Anisotropic Flows And Their Fluctuations And Event Plane Correlations In Relativistic Heavy Ion Collisions

Quantum Chromodynamics (QCD) predicts that new matter could be created at extremely high temperature and high baryon density environment.This new matter is composed of a large number of quarks, anti-quarks and gluons, called Quark gluon Plasma (QGP), which is thought to be present at the early stage of Big Bang. All these years, people did not observe the presence of free-quark,QCD notes that quarks and gluons are confined within hadrons via strong interactions. In order to make quarks break through hadron bound, T.D. Lee et al. proposed that heavy ion collision experiments can create high temperature and high-density environment like the early stage of Big Bang, the quark-gluon plasma may be produced in such an environment.From the high-energy heavy ion accelerator (Bevala) at Lawrence Berkeley Lab to the Super Proton Synchrotron (SPS) at CERN and Alternating Gradient Synchrotron (AGS) and Relativistic Heavy Ion Collider (RHIC) at Brookhaven Lab and the Large Hadron Collider (LHC) at CERN, people have been trying to explore this new matter produced in relativistic heavy ion collisions.In non-central nucleus-nucleus collisions, the overlap region is like an ellipsoid shape. The collision geometry in the configuration space is anisotropic. Due to the interactions among the constituents, the system expands rapidly, the initial geometric anisotropy is converted into momentum space anisotropy. We define the the second-order coefficient (v2) from the distribution of final state particles Fourier expansion as elliptic flow parameter. Elliptic flow is formed early in a collision, therefore, it is closely related with the dynamics of system. RHIC experiments found that the performance of elliptic flow at low transverse momentum area appears the order of quality, which is predicted by hydrodynamics. This means that this new matter is a low viscosity "liquid" fluid system. At intermediate transverse momentum region, we observe number of constituent quark scaling in elliptic flow, which indicates that the confinement states are achieved by collisions and collective motion is formed at parton level. Because of configuration fluctuations, the distribution of nucleons at transverse area may lead to a triangular shape, the initial geometric anisotropy is transfered into momentum space anisotropy via the interactions among the constituents, we define the third-order coefficient (v3) from the distribution of the final state particle Fourier expansion as the triangular flow parameter. Triangular flow plays an important role in obtaining early physical information. Studies have shown that the triangular flow is more sensitive to the viscosity of system, and contribute to the ridge and double-peak structure of two particle correlation. Therefore, study of triangular flow in relativistic heavy ion collisions is also important. Based on AMPT string melting model, we analyze the triangular flow in different collision systems, elliptic flow fluctuations and event plane correlations.In this paper, we first studied triangular flow in Au+Au and Cu+Cu collisions. The results shows that the triangular flow is not sensitive to the collision centrality and collision system, which is obviously different from the behavior of the elliptic flow. After removing the initial geometrical effect, larger values of vn/εn are observed with more central collision, which indicates stronger collective motion with more central collisions; With the same conditions, v2/ε2is significantly larger than v3/ε3. This indicates that the conversion efficiency of s2to v2is lager than that of ε3to v3. At low transverse momentum region, the distribution of the triangular flow meets with the order of quality in both Au+Au and Cu+Cu collision. This means that although the formation mechanism of triangular flow is different from elliptic flow, it also performs hydrodynamic behavior.Then, we systematically studied the sources of elliptic flow fluctuations. With a fixed impact parameter and a given ε2, the distribution of event-by-event elliptic flow shows a great divergence. This suggests that event-by-event elliptic flow fluctuations is not entirely caused by the ε2fluctuations, there should be another fluctuation sources. Besides statistical fluctuations, there may be other sources of dynamical fluctuations. We found that the configuation fluctuations are as important as ε2fluctuations to the elliptic flow development. Quantum fluctuations of the interactions among the constituents may also contribute to the elliptic flow fluctuations. The results shows that elliptic flow fluctuations caused by parton interactions, hadronizations and hadron rescatterings are negative. This indicates that these processes will make elliptic flow distribution narrow. If this conclusion exists in the real data, then the hydrodynamics calculations may overestimate the elliptic flow.Finally, we studied the event plane correlation from forward and backward pseudorapidity region. It shows that forward and backward triangle flow event planes are anti-correlated, which may root in the initial collision geometry (upward and downward triangle shape). We found that the correlation strength corrected by event plane resolutions decreases with increasing pseudorapidity gap. This indicates that the event-plane decorrelation is not simply due to the degrading event-plane resolution, but physics—the event planes from forward and backward pseudorapidities are indeed different.Event plane decorrelation may call into question the pseudorapidity gap method of measuring azimuthal anisotropy. Pseudorapidity gap can simultaneously reduce the non-flow contributions and enhance event plane decorrelation. The event plane decorrelation has been overlooked for long time. Therefore, the pseudorapidity gap method may underestimate the anisotropic flow. If the event plane deorrelation exists in the experimental data, the QGP viscosity will be overestimated. QGP may be more "perfect" than we think.