Study On The Energy Dependence Of The Lund Fragmentation Function In A Multiphase Transport Model

At the beginning stage of the big bang, an extremely hot and dense nuclear matter will be created. In such condition, quarks, antiquarks, and gluons would interact with each other as a new state, the quark-gluon plasma(QGP). Our world was formed after the hot universe cooling down. Quantum chromodynamics theory also predicted that the quarks and gluons can exist in an unconfined state, called quark gluon plasma. The QGP is a plasma in which quarks and gluons can move in an extended volume without being restricted to the hadron size. The relativistic heavy-ion collision provide us an opportunity to study the origin of the universe and microscopic world. The relativistic heavy-ion collider (RHIC)at Brookhaven National Lab in US can accelerate AuAu nuclei up to center of mass energy of 200 GeV per nucleon pair and that for proton is 500 GeV, and the large hadron collider(LHC)at CERN can provide center of mass energy 5.5 TeV for PbPb collisions. The experimental results indicated that the new state of nuclear matter, quark gluon plasma, does exist.In order to study the evolution process of the relativistic heavy ion collision-s, different models were developed, such as thermal model, hydrodynamic model,cascade model, Boltzman-Uehling-Uhlenbeck model, quantum molecular dynamics model and some other classic models. A multiphase transport models (AMPT) is a hybrid model which combined multiple models together. AMPT model contains four stages: initial conditions, parton cascade, hadronization, and hadron cascade.In the process of hadronization, longitudinal momentum f(z) is described by Lund symmetric fragmentation function, the function equation contains the parameters a,forms, a is similar with "amplitude". Because of the different initialization conditions and hadronization methods, the AMPT model has two versions - the default version and string melting version. If the initial parameter a = 0.3 for (?)= 2.76 TeV in PbPb collision and a = 0.55 for (?)= 200 GeV in Au+Au collision, AMPT can describe the production and elliptic flow for different particles species when b=0.15 and parton cross section is setted to 3 mb. Under different collision energies,the best fitting parameter a could be different. So the energy dependence of parameter a is important in AMPT-SM model.In this thesis, we performed our calculations with AMPT-SM model under five collision energies,39/27/19.6/11.5/7.7 GeV for Au+Au collision. We set parameter b as 0.15, parton cross section as 3 mb, then scan the initial parameter a to get the values of dN/dy of different particles. By using chi-square fitting, we get the best fitting parameter a=1.96/2.62/3.74/2.25/2.75 corresponding to different energies of 39/27/19.6/11.5/7.7 GeV. At last, we find that the best fitting parameter a decrease with the energy increase.