| Modern physics has been continuously developing since the 19th century,rigorous and scientific theories have taught us that quarks and leptons constitute the mysterious and ever-changing physical laws of nature through electromagnetic interaction,weak interaction,strong interaction,and gravitation interaction.The strong interaction is the strongest of the four interactions and plays an essential role in establishing the basic composition of the micro world and how the laws of interaction and motion work between particles.The study of strong interactions has led to the development of the strong interaction quantum field theory,also known as Quantum Chromodynamics(QCD)theory,which is based on two fundamental components,namely quarks and gluons,that are tightly bound inside hadrons and cannot exist independently.The lattice QCD theory predicts that the transition from the quark-gluon confined hadron matter phase to the deconfined Quark Gluon Plasma(QGP)phase is a smooth transition at high temperature and low baryon density regions.However,the phase transition occurring at low temperature and high baryon density regions is a first-order phase transition with an endpoint at the boundary of the first-order phase transition,called the QCD Critical Point(CP).Currently,detecting and determining critical points is a hot and frontier topic in high energy experimental and theoretical physics.Recent theories suggest that light nuclei production based on the nucleon coalescence model is closely related to the local baryon number density of system evolution,which predicts that light nuclei in the final state will carry information about the phase transition of nuclear matter,and light nuclei measurements in heavy ion collisions will also serve as an effective probe to explore the QCD phase structure.Light nuclei are relatively stable nuclei consisting of 2-40 nucleons,also known as nucleon clusters.They are important objects of study in both low-and intermediate-energy nuclear physics as well as high energy physics.After more than half a century of research and development,many theories have been developed to try to understand the production mechanism of light nuclei in heavy-ion collisions.Among them,the nucleon coalescence model and the thermodynamic statistical model have been relatively successful,but there is still no definitive answer to this question.In recent years,a theoretical proposal has been put forward that the nuclear compound yield ratio(Nt×Np/Nd2)light nuclei production based on the nucleon coalescence mechanism,is directly connected to the local neutron density fluctuation in the final state system of heavyion collisions.This fluctuation can be utilized as an effective signal for the phase transition of nuclear matter.When the system undergoes a phase transition or approaches the critical point,the correlation length of the system diverges,causing inhomogeneity of the local baryon number density in the system.As the nucleon coalesce into light nuclei,the information related to the fluctuation of baryon number density is directly carried in the light nuclei yield.Based on the nucleon coalescence model,the yield of light nuclei strongly depends on the system volume and freeze-out temperature,so the yield of individual light nuclei is very dynamic and is not an effective observable to understand the dynamical evolution of the system.Otherwise,the compound yield ratio Nt×Np/Nd2 of light nuclei can cancel the overwhelming density and volume effects,and the possible nucleon density fluctuation hidden in the light nucleus yield has the chance of coming to the surface.The Relativistic Heavy Ion Collider(RHIC)at Brookhaven National Laboratory in the United States is one of the large-scale experimental facilities for high-energy heavy-ion collisions in the world.Among them,the STAR experiment is dedicated to the experimental study of quark-gluon plasma properties and QCD phase structure under high temperature and high density conditions.The STAR detector consists of multiple particle detectors with different functions.Its core tracking section includes a cylindrical Time Projection Chamber(TPC)and a Time-of-Flight detector(TOF),which provide full azimuthal coverage and a large central rapidity region,as well as excellent particle identification capability.Since 2010,the STAR Beam Energy Scan(BES)program has collected data from Au+Au collision at center-of-mass energy range from 3 to 200 GeV,corresponding to a baryon chemical potential of 750 to 25 MeV.This energy range covers a wide range on the QCD phase diagram,providing an effective approach to experimentally study the QCD phase diagram.This thesis mainly introduces:(1)the production of triton in Au+Au collisions at(?)11.5,14.5,19.6,27,39,54.4,62.4,and 200 GeV;(2)the extraction of the primordial proton transverse momentum spectra and integrated yields at all energies except 200 GeV;(3)the measurement of the nuclear compound yield ratio Nt×Np/Nd2 from 7.7-200 GeV.Enhancements in the yield ratios relative to the coalescence baseline are observed in the 0%-10%most central collisions at 19.6 and 27 GeV,with a combined significance of 4.1σ.The enhancements are not observed in peripheral collisions or model calculations without critical fluctuation.Theoretical predictions suggest that this non-monotonic behavior is closely related to the phase transition of nuclear matter and the QCD critical point,but there is still no definitive answer.This study systematically investigates the production of light nuclei in the first stage of the STAR BES-I,obtaining important experimental data for the measurement of heavy-ion collisions,including triton yields,primordial proton yields,and yield ratios within a wide energy range.The proposed new observations and experimental measurements provide fresh insights into the mechanisms of light nuclei production in heavy-ion collisions and the understanding of the QCD phase diagram.The thesis focus on the measurement process of triton production,proton weak decay correction,and the ratio of light nuclei in the STAR BES-I.The relevant analysis methods,correction details,model comparisons,and discussions of physical significance will be presented one by one.The paper consists of the following chapters:Chapter 1 provides an introduction to the background of modern physics and the research motivation of this subject.Chapter 2 introduces the RHIC-STAR experimental setup,as well as the detectors and detection methods used in our study.Chapter 3 describes the measurement of triton in the STAR BES-I,providing a detailed explanation of the data analysis process and the final triton yield obtained.Chapter 4 addresses the weak decay of strange particles on the published proton yield of STAR,outlining the correction process,and providing the yield of primordial protons and the fraction of proton feed-down.Chapter 5 presents the final results of our measurements and relevant discussions.Finally,Chapter 6 provides a brief summary and outlook.Researching the production of light nuclei in heavy-ion collisions is of significant scientific importance as it helps to understand the mechanisms behind their formation and explore the QCD phase diagram. |
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