Study Of A Multi-Phase Transport Model In Relativistic Nuclear-Nuclear Collision

In high energy heavy-ion collisions,a new form of strongly interacting QCD matter with parton degree of freedom which is called the Quark Gluon Plasma(QGP)is created in the experiments at Relativistic heavy-ion collider(RHIC)and Large hadron collider(LHC).One of the main goals of the field is to explore the properties of QGP.Theoretical models including transport model,hydrodynamic model and hybrid model are powerful tools for us to achieve the goal.A multi-phase transport model is developed to simulate the whole phase space evolution of heavy-ion collisions.It can reasonably describe multiple physical observables in high-energy heavy ion experiments,and certain predictions based on AMPT model have also been experimentally verified.Therefore,it is widely used in the physical research in the field of relativistic heavy ion collisions.AMPT model contains mainly four parts:fluatuating initial condition,parton scattering,hadronization and hadronic interactions.The code of AMPT model was released online around 2004;with a large amount of experiment data becoming available in the new era of RHIC and LHC,the AMPT model needs to be improved for those corresponding energies.In this thesis,we present the main developments including the parton distribution function(PDF)and nPDFs,the local scaling of the two key parameters in the initial condition and the improvement on heavy flavor productions in the AMPT model.The elliptic flow splitting due to the existence of directed flow is also presented.The initial condition of the AMPT model is based on the HIJING event generator,which employs the Duke-Owens parton distribution function set I for the free proton and a schematic nuclear shadowing parameterization.It is well known that the Duke-Owens PDFs were obtained at a time when large array of experimental data were not available for the global fitting.The gluon and quark distributions at small x region were under-estimated by the Duke-Owens PDF which would lead to significant uncertainties on the heavy flavor and/or high pT observables.Therefore,we update the AMPT model with modern PDF(CTEQ6.1M set)and nuclear modification functions(eps09s NLO).After retuning the key parameters p0 and σsoft in the two component initial condition model from the experimental data of total and inelastic cross section in pp or pp collision from(?)～4GeV to 13TeV,we then use the updated AMPT model to study the particle production in both pp and AA collisions.With fixed Lund symmetry fragmentation parameters aL=0.8 and bL=0.7GeV-2,the updated AMPT model can well describe the dN/dy and pT spectra of charged particles in pp collisions at(?)from 23GeV to 13TeV.The updated AMPT model can also provide a reasonable description of the particle production in AA collisions after we introduce a nuclear scaling of p0 that was motivated by the color glass condensate.It is found that certain key parameters in the initial condition of the AMPT model need quite different values to reasonably describe the particle yields and pT spectra for pp and central A A collisions.Therefore,we scale the two parameters p0 and bL in the initial condition of the AMPT model with local nuclear thickness functions from the two colliding nuclei.This allows the model for the first time to describe the system size and centrality dependence of nuclear collisions self-consistently.Using the parameters from pp collision and local scaling,the improved AMPT model can describe the particle production in pp and AA collisions at RHIC and LHC.It can also describe the centrality dependence of<pT>of charged particles below 2GeV/c.It works similarly well for small collision systems such as p+Pb,Cu+Cu and Xe+Xe collisions.Heavy flavor particles,due to their relatively large quark mass,are one of the most powerful probes to study the properties of the deconfined nuclear matter.Here in this thesis we also improved the heavy flavor description in the AMPT model.In addition to the new PDF and nPDFs,we removed the transverse momentum cutoff for the initial heavy quark production and included the heavy quark cross section in the total minijet cross section in the HIJING two component model.We find that the AMPT model with these updates can provide a much better description of heavy flavor hadrons yield and transverse momentum spectra in comparison with the experiment data.Recently the splitting of elliptic flow at finite rapidity has been proposed as a result of the global vorticity in non-central relativistic heavy-ion collisions.By employing the updated AMPT model which included the global vorticity and flow fluctuations,we confirm the leftright splitting of the elliptic flow at finite rapidities.However,instead of the global vorticity,we find that this elliptic flow splitting is due to the existence of finite directed flow with the splitting magnitude～8ν1/3π.As a result,the splitting vanishes at zero transverse momentum,and the magnitude and sign may have non-trivial dependence on pT,centrality,collision energy,and hadron species.Since the left-right splitting of the elliptic flow is a combined observables from the ν1 and ν2,it will benefit studies of the three dimensional structure and the dynamics of QGP.The AMPT model with these improvements is expected to serve as a better tool when extracting the properties of hot and dense matter produced in relativistic heavy ion collisions.The applicability of AMPT is also expanded,since it can provide better description of heavy flavor productions.The updated AMPT model can be used to study the experiment observables such as collective motion,magnetic field and vorticity et,al.