| Quantum Chromodynamics(QCD),the fundamental theory that governs strong interaction,has predicted the existence of the Quark-Gluon Plasma(QGP).The QGP is a state of the strongly interacting matter in which quarks and gluons are no longer confined to volumes of hadron.Experiments at the Relativistic Heavy Ion Collider(RHIC)and Large Hadron Collider(LHC),have provided evidences for the creation of QGP matter in the early 21st century.Since the discovery of the QGP,physicists have been investigating the phase transition between hadronic phase and QGP phase,and the corresponding QCD phase diagram which can be mapped and displayed into a two dimensional plane of temperature(T)versus baryon chemical potential(μB)Lattice QCD calculations predicted a crossover transition from hadronic matter to QGP at vanishing μB.At large μB,QCD-based model calculations suggested that the phase transition is of the first-order.One essential feature of the QCD phase diagram is the critical point(CP),where the first-order phase transition boundary terminates.Nowadays,many researches worldwide are working towards finding the the possible CP in heavy-ion collisions,especially the Beam Energy Scan(BES)program at the RHIC-STAR.Within the framework of intermittency analysis,a search for critical density fluctuations is ongoing to locate the possible CP in the QCD phase diagram.Based on the Ising-QCD calculations,the density-density function has a power-law,or self-similar,structure which gives rise to large density fluctuations in heavy-ion collisions.Such fluctuations can be probed via an intermittency analysis by utilizing the scaled factorial moments(SFMs).The intermittency termed as big bursts from small region(cells)of the phase space,appears as a power-law(scaling)behavior of SFMs.The strength of intermittency can be quantified by the intermittency index(φq)extracted from the power-law behavior of SFMs on the number of partitioned cells(M),and by the scaling exponents(ν)obtained from the power-law behavior of higher-order SFMs on the second-order one.Over the last decade,the NA49 and and the NA61/SHINE experiments have been searching for the critical point by performing intermittency analysis in heavy-ion reactions of various sizes and collision energies.Meanwhile,various models have investigated the intermittency under various fundamental mechanisms in heavy-ion collisions,such as the UrQMD(ultra-relativistic quantum molecular dynamics)model with hadronic potentials.In this thesis,we present the first measurement of intermittency in heavy-ion collisions at RHIC,and show the collision energy and centrality dependence of SFMs and intermittency exponents for identified charged hadrons in Au+Au collisions from the STAR experiment.The data presented here have been obtained from Au+Au collisions at(?)=7.7,11.5,14.5,19.6,27,39,54.4,62.4,and 200 GeV,recorded by the Solenoidal Tracker at RHIC(STAR)experiment from 2010 to 2017.These energies correspond to baryon chemical potential ranging from 20 to 420 MeV at chemical freeze-out in the QCD phase diagram.The mixed event method is applied to eliminate background contributions,and the cell-by-cell method is proposed to the application of efficiency corrections on SFMs.The SFMS of identified charged hadrons are analyzed at mid-rapidity and within the transverse momentum phase space,and can be calculated up to the sixth order with the range of number of cells M2=1-1002.We observe a power-law behavior of scaled factorial moments in Au+Au collisions and a decrease in the extracted scaling exponent(ν)from peripheral to central collisions.The ν is consistent with a constant for different collisions energies in the mid-central(10-40%)collisions.Moreover,the ν in the 0-5%most central Au+Au collisions exhibits a non-monotonic energy dependence that reaches a possible minimum around(?)=27 GeV.The non-monotonic energy dependence of ν agrees with those from other several measurements,such as,the net-proton kurtosis,the slope of directed flow for net-proton,and the ratio of light nuclei production.We use the cascade UrQMD model to estimate the contributions from non-critical fluctuations on SFMs and intermittency exponents.It is found that the power-law behavior is not valid when the background contributions are subtracted,in the original UrQMD model.Moreover,a Critical Monte Carlo(CMC)model which can simulate critical intermittency driven by density fluctuations,is used to study the property of intermittency in heavy-ion collisions.Critical fluctuations from the CMC model have been incorporated into the UrQMD model to describe and understand the intermittency measured in experiments.By comparing the UrQMD+CMC model results with those from the STAR data,it is found that the value of a calculated scaling exponent falls in the range of the experimental measurement when 1-2%signal of intermittency fluctuations is added into the UrQMD sample.The thesis is organized as follows.Chapter 1 is an introduction of the QCD phase diagram and the critical point,and the framework of intermittency analysis.In chapter 2,we describe the methods of background subtraction and efficiency correction for the experimental analysis.We shortly introduce the STAR experiment at RHIC in chapter 3.The analysis detail of the STAR data is given in chapter 4.The results from the STAR data are discussed in chapter 5.In chapter 6,we show the result of intermittency from the UrQMD,CMC,and hybrid UrQMD+CMC models,respectively.Finally,a summary and outlook is given in chapter 7. |
