Measurements Of ? Elliptic Flow In Au+Au Collsions At (?)=27GeV With RHIC/STAR Detector

At the early stage of high energy relativistic heavy ion collisions,a hot and dense strong interaction medium was created,called quark gluon plasma(QGP).Experimentally,we can study the properties of the QGP medium by measuring the azimuthal anisotropy of the final particles produced in heavy ion collisions with respect to the reaction plane.The collision parameter b refers to the lateral distance of the collision nucleus center.In non-concentral collisions(b?0).The area involved in the collision is "almond-shaped",which causes the system is anisotropic in the initial coordinate space.However,as the system ex-pands,the anisotropy of the initial coordinate space will be transformed into the anisotropy of the momentum space of the final state.The elliptic flow v2 is the second coefficient of the Fourier expansion of the azimuth distribution of the emitted particles in the momentum space relative to the reaction plane.The elliptic flow comes from the early stage of the collision and can provide the dynamic information of the early stage of the collision,in-cluding the pressure gradient,the effective degree of freedom,the degree of thermalization,and the equation of state of matter produced in the early stage.However,early dynamic information might be obscured by later hadronic strange rescatterings.Strange hadrons,es-pecially multi-strange hadrons,which are produced in the early stages of collisions.Their less sensitive to hadronic rescatterings in the late stage of collisions,as their freeze-out temperatures are close to the phase transition temperature and their hadronic interaction cross sections are expected to be small.It can be used as an ideal probe for studying the properties of hot and dense materials.In this thesis,we measured the elliptic flow of multi-strange hadrons ? in Au+Au col-lisions at(?)=27 GeV with STAR detector.Firstly,it is found that the elliptic flow v2 has transverse momentum dependence.In the low transverse momentum region(pT?2 GeV/c),the elliptic flow of ? particle increases with the increase of pT,and hydrodynamics predicts that the elliptic flow of ? particle will continue to increase.However,the experi-mental results show that in the higher transverse momentum region,the elliptic flow starts to moderate with the increase of transverse momentum,and then reaches saturation.This indicates that the elliptic flow of ? particles in the region of medium and high transverse momentum(pT>2 GeV/c)cannot be described by hydrodynamics,but follows the num-ber of constituent quark scaling(NCQ).This scale indicates that the collision system has reached local thermalization,and the degree of freedom of partial molecules is dominant at this time,indicating that the collective flow is generated in the partial molecular stage generated in the heavy ion collision.Therefore,the NCQ of the elliptic flow of particles is a strong evidence for the formation of QGP.Secondly,it is found that the elliptic flow v2 has centrality dependence,and the value of the elliptic flow in the edge collision is higher than that in the central collision,which indicates that the initial spatial anisotropy will affect the spatial anisotropy of the final momentum.Finally,the result of the difference between the elliptic flow of the ?-particle and its antiparticle?+found in this paper is consistent with the energy-dependent trend of the difference between the elliptic flow of other positive and negative particles.It is also one of the most notable features in RHIC-STAR BES-I elliptic flow measurement.