Measurements Of Strangeness Production And Correlation Function In Au+Au Collision At (?)=3 GeV With STAR At RHIC

The Solenoid Tracker At RHIC(STAR)is a major experiment at the Relativistic Heavy-Ion Collider(RHIC)located at Brookhaven National Laboratory(BNL)in Upton.The main goal of the RHIC STAR experiment is to study the properties of Quark-Gluon-Plasma(QGP)and quantum chromodynamics(QCD)phase diagram at extreme conditions of high temperature and/or high density by colliding heavy ions at ultra-relativistic speed.The RHIC Beam Energy Scan(BES)program covers a wide range of energies to explore the transition from a hadronic-dominated phase to a partonic-dominated one.Of particular interest is the high baryon density region which is accessible through the STAR Fixed-Target(FXT)program,which has extended the energy reach from(?)=7.7 GeV down to 3.0 GeV,and extends the coverage of baryon chemical potential μB to 750 MeV.In this thesis,we mainly focus on FXT(?)=3 GeV Au+Au collision dataset taken in 2018,it has the best midrapidity acceptance coverage,and this is the highest baryon density region RHIC has reached.The measurement of v2 in 3 GeV Au+Au collisions has shown the breaking of number-of-constituentquarks(NCQ)scaling,which indicates no partonic collectivity and implies that baryonic interactions characterize the produced medium in such collisions.The baryon-dominated systems created in this energy may be similar to the properties of matter expected in the inner core of neutron stars.Therefore,studying heavy-ion collisions at these energies allows deducing information on the nuclear Equation of State(EOS)relevant to the stability of neutron stars.The strange quark mass is comparable to the QCD renormalization scale(ΛQCD～200 MeV),therefore strange quark dynamics plays an important role in understanding the EOS of QCD matter,particularly in the high density region.Indeed,the structure of the innermost core of neutron stars is still unknown and hyperons could appear in such environments depending on the hyperons-nucleons(YN)and hyperons-hyperons(YY)interactions.The kaons(such as KS0 mesons)-nucleons(KN)and hyperons(such as Ξ-)-nucleons(YN)interactions are important criteria for studying and testing various effective models of QCD.Precise measurements of strange particle spectra and yield ratios at low collision energies can give input on their production mechanisms.The production yield could be explained based on statistical thermal models.It has been argued that at lower energies,the strangeness number needs to be conserved locally on an event-by-event basis described by the canonical ensemble.Meanwhile,the different particles’transverse momentum distributions reflect the matter’s kinetic properties at kinetic freeze-out after elastic collisions have ceased.The femtoscopic correlations between nucleons and strange particles can be directly used to constrain the potential or interactions.It is based on a study of the two-particle correlation function,where final-state effects among specific particle pairs induce a signal that is characterized by the space-time extent of the particle emission source.Recent studies demonstrate that the strong interaction can be studied from this observable if the characteristics of the emission region are sufficiently constrained.An attractive p-Ξinteraction has been observed in p-Pb and p-p collisions at ALICE.Measurements of such correlation function at 3 GeV will help to understand the medium properties and collision dynamics in the high baryon density region.We will report multi-differential measurements of strange hadron yields in(?)=3 GeV Au+Au collisions from the STAR experiment.KS0 meson and Ξ-hyperon are measured through their hadronic decay channels:KS0→π+π-and Ξ-→Λ(pπ-)π-.After correcting for the detector acceptance and efficiencies,the rapidity and centrality dependence of the transverse momentum spectra(pT),rapidity density distributions(dN/dy),and average transverse momentum(<pT>)are presented.We have observed that:(1)The average transverse momenta of strange particles(Ξ-,Λ,Φ,KS0,K-)are ordered by their mass;(2)The consistent centrality dependence of the total yield of S=1 hadron is observed while Ξ-,Φ,and proton deviate from the trend;(3)Strange particle ratios(Φ/K-,Φ/Ξ-,Λ/p,Ξ-/A)favor the canonical ensemble model for strangeness production in such collisions.Those results are compared to the Ultrarelativistic Quantum Molecular Dynamics(UrQMD)transport model calculations,which show that:(1)Overall,the model does not describe the absolute yields of all particles well,but it can roughly describe the shape of the Λ/p ratio;(2)UrQMD could reasonably describe our measured Φ/K-ratio at 3 GeV and the increasing trend of Φ/Ξ-at lower energies by including the resonance decays.The precise measurements can help to constrain the model calculations better.Strange hadron yields presented here,together with other particles measured at the same energy(π,K,p,Φ,and Λ),have been used for chemical equilibrium models to determine the chemical freeze-out temperature Tchem and baryon chemical potentialμB.The result shows the model can not describe the yield of all particles simultaneously,especially forπ yield.The blast-wave with Boltzmann-Gibbs statistics is used for fitting the pT spectrum to extract kinetic freezeout parameters,the freezeout temperature Tkine,and flow velocity We observe that the Tkine of p,A,and KS0 are significantly lower than higher energy collisions.The result may imply a different EOS at freeze-out in 3 GeV collisions compared to those at higher energies.Meanwhile,the first measurement of p-Ξ-correlation function in heavy-ion collisions at high baryon density from the STAR experiment is reported.Detailed studies of the femtoscopic correlations are performed employing the single ratio technique of particles from the same event to those from different events,together with several other residual correlations due to feed-down sources and final state interactions of misidentified particles to extract the desired information.The measured p-Ξ-correlation has a large statistical uncertainty due to limited statistics of Ξ-hyperons.The UrQMD simulation is used to determine the p-Ξ-source at this energy.Given an interaction potential between two hadrons as a function of their relative distance,a Schrodinger equation can be used to obtain the corresponding wave function and hence also predict the expected correlation function.The UrQMD incorporated with an attractive femtoscopy effect calculated by lattice QCD collaboration delivers a good description of the correlation signal.In this thesis,we mainly focus on the fixed-target 3 GeV datasets taken in 2018.More than 2 billion events of 3 GeV Au+Au collisions were collected in 2021,allowing us to precisely and systematically study the strange production and strangeness correlation.