| According to Quantum Chromodynamics(QCD),hadronic matter undergoes phase transition in an extremely hot and dense environment,forming a new form of matter,called the Quark-Gluon Plasma(QGP).A major focus of high energy nuclear physics is to search for the critical point and the phase boundary between hadronic phase and QGP phase in QCD phase diagram.To explore the QCD phase diagram,the STAR experiment at the Relativistic Heavy Ion Collider(RHIC)started Beam Energy Scan(BES)program in 2010.It is important to accurately identify the region accessed by collision experimental in the phase diagram.We can obtain the corresponding temperature(T)and baryon chemical potential(μb)of the chemical freeze-out and kinetic freeze-out of QGP evolutions of different collision energies((?))from the transverse momentum(pT spectra and yield ratios of the produced particles.The T-μb curve of the collision evolution can thus be plotted in the QCD phase diagram,this will be a powerful tool for exploring the QCD phase diagram.In 2017,the STAR collaboration completed experimental measurements of hadron(π±,K±and p(p))spectra from BES-Ⅰ at RHIC.However,due to the limited statistics of BES-I,the measurements only covered the region of low transverse momentum(pT<2 GeV/c).The statistics of BES-Ⅱ increased exponentially compared with BES-Ⅰ.For example,at Au+Au(?)GeV,the total number of events in BES-Ⅱ is 10 times that in BES-I.High statistics provides opportunity for measuring high transverse momentum of the hadron spectra and yield ratios,to extract the properties of chemical and kinetic freeze-out more precisely,also test and reveal particle production mechanism in regions of high transverse momentum.This thesis conducted measurements of high transverse momentum hadron(π±,p(p))spectra at mid-rapidity in Au+Au collisions at BES-Ⅱ(?)GeV at the STAR experiment.π± spectra measurement reached PT=5 GeV/c.p(p)spectra reached pT=4 GeV/c.The yield ratio of π-/π+,p/p,p/π+ +and as a function of the transverse momentum pT are also reported.We find that the variations of π-/π+ and p/p with transverse momentum have no obvious centrality dependence within the range of considered systematic errors,and show a decreasing trend with increasing transverse momentum.p/π+ and p/π-of high transverse momentum(pT>2 GeV/c)along with the change of transverse momentum decrease from the central to the peripheral collision,and first increase then decrease with the increase of transverse momentum.The measured results of yield ratio for p/p,p/π+ and p/π-in Au+Au collisions varies with transverse momentum,show obvious collision energy dependence.This is the first measurement of its kind at BES-II,which is crucial for accurately evaluating chemical and kinetic freeze-out parameters,and provides important experimental data for understanding the partonic dynamics(hard or semi-hard processes)taking place in the early stage of the collision and validating the related theoretical models.The second part of this thesis applied the blast-wave model to complete a systematic study of kinetic freeze-out properties of the system created in relativistic heavy ion collisions at RHIC and LHC energy and to study their collision energy and centrality dependence.We comparatively study the fitting results of the transverse momentum spectra of hadrons using Boltzmann-Gibbs blast-wave(BGBW)model based on local thermal equilibrium and Tsallis blast-wave(TBW)model based on nonextensive Tsallis statistics.The fitting results show that,TBW fits better than BGBW,with an increasing q value which characterizes the degree of non-equilibrium as collision energy increases,especially in peripheral collisions,which indicates that the non-extensive statistics describe the generated system in collision better.The TBW model shows that,as the collision energy increases,The kinetic freeze-out temperature first increases and then decreases,while the average radial flow velocity<β>and the nonequilibrium parameter q both increase.For centrality dependence,the average transverse radial flow velocity decreases and the degree of non-equilibrium q increases from central to peripheral collisions in TBW model.We also find that strange hadrons have a higher kinetic freeze-out temperature than that for light hadrons.The strange hadrons approach equilibrium more quickly from peripheral to central A+A collisions than non-strange hardons.Finally,we discussed the impact of high π±,p(p)transverse momentum spectra measurements on the determination of chemical freeze-out and kinetic freeze-out properties.For kinetic freeze-out fitting results with our new measurements of the high transverse momentum spectra,the average radial flow velocity decreases,and the kinetic freeze-out temperature increases in the intermediate centrality region while the degree of nonequilibrium increases significantly.This finding indicates that even at the most central collision,non-equilibrium processs needs to be considered in high transverse momentum region.For chemical freeze-out,we found that the effect of yield ratios in the high transverse momentum region on the overall dN/dy value is much smaller than that in the low transverse momentum region,and the effect on the dN/dy value at mid-rapidity of the particles is no more than 5%.Finally,we also com pare and discuss the yield ratio change with the transverse momentum with the model results of different particle production mechanisms which including quark coalescence/recombination in hadronization,baryon junctions and jet fragmentation.In this thesis,measurements of the high transverse momentum π± p(p)spectrum at mid-rapidity in(?)GeV Au+Au collision,and the model analysis of kinetic freeze-out parameters from RHIC BES to LHC energy region,provide experimental basis and model attempt for understanding the effect of high transverse momentum hadron spectrum on chemical and kinetic freeze-out properties,the test of related theoretical models also promote the study of the particle production mechanism at the region of high transverse momentum in heavy ion collisions. |
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