Study On Collective Flow Of Hypernuclei(_Λ~3H And_Λ~4h) In High Energy Nucleus-Nucleus Collision

The Quantum Chromodynamics（QCD）theory predicts that under extreme con-ditions of high temperature or high density,quarks and gluons inside hadrons may break the confinement and form Quark-Gluon Plasma（QGP）.Relativistic heavy-ion collisions are used as an effective means to produce QGP in laboratory conditions.The Relativistic Heavy Ion Collider（RHIC）at Brookhaven National Laboratory（BNL）and the Large Hadron Collider（LHC）at CERN have provided a wealth of experimental evidence for the production of QGP in high energy heavy-ion collisions.It is important to search for the phase boundary and QCD critical point for understanding the properties of strong interactions,the matter state in the early universe and the internal structure of compact stars.Hypernuclei are bound states formed by hyperons and nucleons,and the study of hypernuclei and their properties has become an important research topic in modern nuclear physics.Due to the short lifetime of hyperons,it is difficult to conduct hyperon-nucleon scattering experiments.Therefore,hypernuclei have become an effec-tive probe for studying hyperon-nucleon（Y-N）interactions.Theoretical predictions suggest that hypernuclei have a large production cross-section in heavy-ion collisions with center-of-mass energies in the range of 3-8 Ge V.Measurements of the collective flow of hypernuclei in such environments may provide experimental evidence for the production mechanisms of hypernuclei in high density regions and Y-N interactions in dense media.The collective flow refers to the Fourier expansion coefficients of emitted particles in momentum space relative to the reaction plane.It characterizes the anisotropy of the early coordinate space converted into momentum space due to the（density）pressure gradient.The directed flow（v₁）is the first harmonic coefficient,which established in the early stages of system evolution.Both transport models and hydrodynamic models indicate that v₁in the mid-rapidity region is highly sensitive to the dynamical information and equation of state（Eo S）of the matter involved in the early collision stages.Measuring the collective flow of hypernuclei in the high baryon density region may help extract information about the Y-N interaction at finite pressure and has important implications for understanding the density-dependent Y-N interaction and the inter structure of compact stars.This thesis mainly introduces the reconstruction ofΛhyperon,_Λ³H and_Λ⁴H,and directed flow measurements in the fixed-target（FXT）mode of the STAR experiment in （?） collisions at the center-of-mass energy(s _NN)of 3 Ge V.These data,containing about 260 million events,were collected in 2018 and are part of the RHIC Beam Energy Scan（BES）program.This energy is the lowest center-of-mass energy that RHIC can reach and corresponds to the highest baryon density region covered by RHIC.In this thesis,we will use the event plane method to measure the directed flow v₁.The first-order event plane is reconstructed using the Event Plane Detector（EPD）,and charged particles（π^-,p,d,³He and⁴He）are identified based on the Time Projection Chamber（TPC）.The KFParticle package based on the Kalman filter method will be used to reconstruct the invariant mass spectra ofΛhyperon,_Λ³H,and_Λ⁴H through the hadron decay channelsΛ→p+π^-,_Λ³H→³He+π^-,_Λ³H→d+p+π^-and_Λ⁴H→⁴He+π^-.After applying the correction of the first-order event plane resolution and detector reconstruction efficiency,we measured the rapidity dependence of the v₁ofΛhyperon and hypernuclei,and compared them with those of p,d,t,³He,and⁴He from the same collisions.The results show that the values of v₁ofΛhyperon,_Λ³H,and_Λ⁴H in the negative rapidity region are negative,and their slopes(dv₁/dy|_y=0)are systematically lower than those of proton and light nuclei with the same mass number.Within uncertainties,the dv₁/dy|_y=0of p and light nuclei shows a linear relationship with their mass number,which means that they follow the baryon number scaling.Similar to them,the dv₁/dy|_y=0ofΛhyperon,_Λ³H and_Λ⁴H also have mass dependence.When fitting their dv₁/dy|_y=0with mass using a linear function,it was found that their slopes are close within uncertainties,indicating that they follow a similar baryon number scaling.Meanwhile,we simulated the production of p,n,andΛhyperons at the same energy using transport models（JAM and Ur QMD）.Both models include Skyrme mean field potential.Then,based on the phase-space distribution of nucleons andΛhyperons,we first formed d and t using the coalescence model,and then formed_Λ³H and_Λ⁴H through d+Λand t+Λ,respectively.Using these two transport models and coalescence model,we calculated the v₁of p,light nuclei,Λhyperon and hypernuclei and qualitatively described the rapidity dependence of their v₁and the mass dependence of dv₁/dy|_y=0.These observations suggest that the main production mechanism of_Λ³H and_Λ⁴H in Au+Au collisions at（?）=3 Ge V is the coalescence of light nuclei（d,t）andΛhyperon.The collective motion of matter in heavy-ion collisions has a direct relationship to the pressure gradient inside dense nuclear matter.This work has completed the first measurement of hypernuclei directed flow,which opens up a new way for the study of Y-N interaction under finite pressure and high density nuclear medium,and may promote the understanding of the equation of state of nuclear matter including strange baryon components and the internal structure of compact stars.