Measurements Of Higher-order Cumulants Of Net-Proton?Proton And Anti-Proton Multiplicity Distributions And Correlation Functions In Relativistic Heavy Ion Collisions

Strong interaction forces(also known as nuclear forces)are one of the four funda-mental interaction forces in nature that bind nucleus(protons and neutrons)to form atomic nuclei and dominate more than 90%of the visible matter in nature.Quantum Chromodynamics(Quantum Chromodynamics,QCD)is a modern theory that describes strong interaction forces.The basic unit of the substance,quarks and glues,is confined to the nucleus by strong interaction forces,so no free quarks and gluons are found in nature.The phase diagram of high temperature and high-density nuclear material is the frontier and hot spot in the field of nuclear physics research.Lattice QCD predicts that at high temperature and low baryon chemical potential,the phase transition between the hadron matter and the quark gluon plasma occurs is smooth crossover,while the model predicts that at the high baryon chemical potential,the phase transition between them is a first-order phase transition.Therefore,if the first-order phase transition does exist,then there must be an end point in the end of the first-order phase transition line to the smooth crossover,which is called the QCD critical point.The experimental confirmation of QCD critical point,will be a milestone in the explo-ration of the phase structure of strong interaction substances,which is of great scientific significance.In order to take a leading position in this potentially significant discovery research direction and make a breakthrough,various countries have built large particle detectors and carried out heavy ion collision experiments(including:RHIC-STAR beam energy scanning experiment in the United States,CBM experiment in Germany,NICA experiment in Russia,J-PARC experiment in Japan and CEE experiment of the external target of CSR in Lanzhou,China),the main physical goal is to study the structure of high temperature and high-density nuclear material phase diagram,search for the critical point.In the first phase of Beam Energy Scan(BES)program,Relativistic Heavy Ion Collider(RHIC)located at Brookhaven National Laboratory(BNL),in the Unites States used the STAR detector to complete data collection of 7.7,11.5 14.5,19.6,27,39,54.4,62.4 and 200 GeV by accelerating heavy ions.This allows us to explore the phase diagram in a broader range.In this thesis,we have finished the measurements of up to the fourth-order cumulants(Cn)of the proton,anti-proton and net-proton multiplicity distributions and correlation functions of(anti-)protons in Au+Au collisions for center of mass energies per nucleon pair,(?)=7.7,11.5,14.5,19.6,27,39,54.4,62.4 and 200 GeV.The measurements are carried out at mid-rapidity(|y|<0.5)and for transverse momentum 0.4<pT<2.0(GeV/c);the measurements of up to the fourth-order cumulants(Cn)of the proton,anti-proton and net-proton multiplicity distributions in Cu+Cu collisions for center of mass energies per nucleon pair,(?)=22.4.62.4 and 200 GeV at mid-rapidity(|y|<0.5)and for transverse momentum 0.4<PT<0.8(GeV/c);the measurements of up to the fourth-order cumulants(Cn)of the proton,multiplicity distributions in Au+Au collisions at Fixed-Target mode(?)=.5 GeV at-2<y<0 and for transverse momentum 0.4<pT<2.0(GeV/c).The various order cumulants Cn and their ratios can be expressed as a function of collision centrality;rapidity,transverse momentum pTand collision energy.We observe a non-monotonic variation of the ratio of C4/C2 with the significance of 3.1? for the most central(0-5%)Au+Au collisions with(?).Transport model UrQMD and Hadron Resonance Gas(HRG)model calculations are carried out in the STAR acceptance to understand the effect of pT acceptance,net-baryon versus net-proton and conservation of net-baryon number.The UrQMD and HRG model calculations of C3/C2 and C4/C2 in Au+Au collisions show a monotonic variation with(?).The collision energy de-pendence of the C4/C2 is consistent with expectations from a QCD based model with critical point.Further,we extract the various order correlation functions of protons and anti-protons from the measured cumulants in Au+Au collisions and find that the large value of C4/C2 for proton distributions in central collisions at(?)=7.7 GeV is due to four-particle correlations.In the fluctuation measurements of conserved quantities in the RHIC's first beam energy scan program,we observed the non-monotonous dependence(3.1?)of the net-proton number four-order fluctuations on the collision energy for the first time,which provides an important experimental baseline for searching for the crit-ical point of QCD phase transition,and also lays the foundation for the high precision measurement of the conservation load in the second phase of beam energy scan and the STAR fixed target experiment at RHIC.This thesis is organized as follows.The first chapter mainly introduced the mo-tivation,the experimental measurements and the presentation of these observables in statistics and probability.In the second chapter,we briefly introduced the structure and function of the STAR detector and its sub-detectors at RHIC.The third chapter mainly introduces the details of experimental analysis,data selection,event selection,particle identification,definition of centrality,multiplicity distribution of net protons and model introduction.The fourth chapter mainly studies the effect of some effects on the re-sults,such as the centrality bin width correction of and the limited detector efficiency correction.In the last chapter,we will present the calculation results of the experiment,including the centroid model and fixed target mode in the Au+Au collisions and the Cu+Cu collisions,and discuss the development prospects of the experiment.