Experimental Study On Strangecharm Meson Production And Proton-proton Correlation Function In Au+Au Collisions At RHIC-STAR

Relativistic heavy-ion collisions generate an extreme environment with high temperature and energy density which is similar to the early universe.A new form of matter,Quark Gluon Plasma（QGP）in which the quarks and gluons are the typical degrees of freedom,is produced under such condition.Studying the properties of QGP is one of the main motivations of Relativistic Heavy Ion Collider（RHIC）at Brookhaven National Laboratory.Charm quarks,which are mainly produced from the initial hard scatterings among partons in the early stage,experience the whole evolution of heavy-ion collisions and are proved to be a good probe for studying the QGP properties.Measurements of strange charmed meson（D_s^±）production can help us understand the hadronization mechanism of charm quark and further extract the thermodynamical coefficiencies of QGP.The STAR experiment at RHIC focuses on two scientific directions:1)studying the properties of quark matter,2)exploring the QCD phase structure.In this thesis,we present the D_s^±production measurements with data from Au+Au collision at√s _NN=200 GeV collected in year 2014 and 2016 at STAR.The Heavy Flavor Tracker（HFT）is designed to measure charmed mesons at STAR.With silicon-pixel technology the hit position resolution of tracks reaches micrometer level,which is crucial to reconstruct the decay vertex and topological variables of charmed mesons.Machine-learning method is used to optimize the cuts of decay topological variables.D_s^±candidates are reconstructed by invariant-mass method with optimized cuts as mentioned afore,and then the raw yield of D_s^±is extracted based on the invariant mass distribution.The tracking and reconstruction efficiencies are calculated by a data-driven simulation,and then used to obtain the corrected D_s^±yield.Based on the measurement of D_s^±production and the published results of D⁰ production,the ratio of D_s^±and D⁰ yield（D_s^±/D⁰）is calculated and compared to those from PYTHIA（p+p collisions）and model calculations with coalescence hadronization mechanism.The D_s^±/D⁰ enhancement relative to PYTHIA calculation is observed for 1<p _T<8 GeV/c,and it shows weak collision centrality and transverse momentum dependence.It indicates the D_s^±hadronization mechanism in Au+Au collisions is different with parton fragmentation hadronization mechanism in PYTHIA.Models including coalescence mechanism of charm and light/strange quarks in the formation of D mesons can qualitatively describe the observed D_s^±/D⁰ enhancement.The experimental results indicate that coalescence plays an important role in D_s^±meson production mechanism.We also present the measurements on proton-proton（p-p）correlation function with～250 M data from Au+Au collision at√s _NN=3 GeV collected in year 2018 under the fixed target mode at STAR.According to the energy loss distribution measured by Time Projection Chamber（TPC）and the particle mass information measured by Time of Flight（TOF）,the protons are selected with high purity and the relative momentum k^*from same event（SE）and mixed event（ME）are calculated.The p-p correlation function is defined as the ratio of k^*distribution from SE to ME.The p-p correlation functions from different rapidity windows and collision centralities are calculated,and the track-splitting and track-merging effects on the correlation function are studied and removed.Based on the phase-space information from a transport model Ur QMD and the correlation calculation provided by CRAB,the p-p correlation functions are simulated event by event.The momentum resolution effect and the secondary decay contribution are investigated based on simulated correlation function.Lednicky-Lyuboshitz（LL）model is used to fit the measured correlation functions,then the source-size r₀,s wave scattering-length f₀ and effective radius d₀ are extracted.The fitting results show that the r₀ from central collisions is larger than that from peripheral collisions.Compared to the forward rapidity region,the r₀ from mid-rapidity is more sensitive to collision centrality.The experimental results indicate that particles emitted from the mid-rapidity region experience sufficient interactions and their space distribution is sensitive to collision centrality.On the other respect,particles from the forward rapidity region experience insufficient interactions and their space distribution is insensitive to collision centrality.In summary,we present the D_s^±production measurements from Au+Au collision at√s _NN=200 GeV and the p-p correlation function measurements from Au+Au collision at√s_NN=3 GeV.The D_s^±production measurements indicate that coalescence hadronization plays an important role in D_s^±meson production.LL model is used to fit the p-p correlation functions,then the source-size r₀,s wave scattering-length f₀ and effective radius d₀ are extracted,and the relevant physical mechanisms are discussed.