| Quark-Gluon Plasma(QGP)is a strongly acting material form existing in the early universe under extremely high temperature conditions.Relativistic heavy ion collisions are usually used to produce QGP.An important research topic in high-energy heavy ion collision physics is the phase transition and properties of strongly acting substances under extremely high temperature and high density conditions.In the low baryon density region of the QCD phase diagram,the transition from hadronic matter to QGP is a smooth transition.In the region of high baryon density,there may be an obvious first-order phase transition boundary,and the end point of the boundary is called the QCD critical point.The experimental confirmation of QCD critical point is of great significance for understanding the structure of visible matter and studying the evolution of the universe.In relativistic heavy ion collisions,light nuclei production provides an effective tool for probing the QCD critical point.A large number of light nuclei are formed at the end of heavy ion collisions,which are detected by detectors,and the temperature and freez-out time of light nuclei production are very sensitive to the phase space density.We can measure the transverse momentum spectra of the light nuclei and estimate the thermodynamic quantities of the system by using some models,such as the temperature(T)and the baryon chemical potential(μB),which are important for the determination of the phase boundary of the QCD phase diagram.At the same time,coalescence parameter BA can express the probability of nucleon coalescence.In thermal model,the value of the coalescence parameter(BA)is inversely proportional to the effective volume(Veff)of nuclei,the larger BA,the smaller Veff.So the study of light nuclei can provide us with information on nucleon association and nucleon density.In addition the phase transition will result in a large baryon density fluctuations,so the yield ratio of light nuclei(Nt × Np/Nd2)is a physical quantity that is very sensitive to the critical point.In this thesis we measure the transverse momentum spectra of proton and light nuclei for various collision centrality and rapidity bins at STAR Fixed-Target Au+Au collisions(?)3.2 GeV.And we calculate the coalescence parameter BA,the particle yield(dN/dy),the average transverse momentum<pT>,the particle ratio(d/p,t/p,3He/p,4He/p)and the yield ratio(Nt × Np/Nd2,N4He×Np/(N3He×Nd))of different centrality and rapidity bins.Compare the results with those of other energies,we find both the particle ratio(d/p,t/p)and the coalescence parameters(B2(d),B3(t),B3(3He))at 3.2 GeV follow the energy dependence trend.But the initial result of yield ratio(Nt × Np/Nd2)shows a deviation of about 2.9σ relative to the coalescence baseline.In addition,the dependence of kinetic freez-out parameters(Tkin,<βT>on centrality and energy is also discussed.Finally,the raw transverse momentum spectra of proton and light nuclei of STAR Fixed-Target Au+Au collisions(?)=3.5 and 3.9 GeV are presented for various centrality and rapidity bins. |
