Cumulants Of Event-by-event Net-Strangeness Distribution In Au+Au Collisions Within An UrQMD Model

Quark-gluon plasma(GQP),a new state of matter,which is believed exists at the early stage of big bang,relativistic heavy-ion collisions or in the core of condensed stars.This state of matter is different with gas,liquid or solid state that we have known and it consists of free quarks and gluons.Studying this matter can help us have a better understanding on the origin of mass and confinement as well as the early universe.The phase transition from QGP to hadronic matter is one of the most important focus at the field of relativistic heavy-ion collisions.The theory of lattice quantum chromodynamics(LQCD)indicates that in the region of zero baryon chemical density the transition from the hadronic phase to quark-gluon phase is a smooth crossover.In the region of large baryon chemical density and low temperature,the phase transition is of first order.There be an end point that we call the critical point at the end of the first order phase transition boundary towards the crossover region.The study of QCD phase structure and critical point are the primary target of relativistic heavy-ion collision experiments.As the high moments of conserved quantities are sensitive to the correlation length which diverges at the critical point and the high moments have non-monotonic behavior and sign change near the critical point.Fluctuations of conserved quantities,such as baryon,charge,strangeness,are sensitive to the QCD phase transition and critical point.Due to the limitation of detector equipment and experimental condition,the net-kaon number is a proxy for net-strangeness number used in heavy ion collision.Since net-kaon number is not a conserved quantity,when studying the fluctuation of net-strangeness we should consider some more strange mesons and strange baryons.Some strange baryons can not be detected and recorded in time in the experiment.In order to have a better understanding on experimental results,we chose a model to study the fluctuation of net-strangeness for reference.The ultra-relativistic quantum molecular dynamics(UrQMD)model is a microscopic transport model based on the hadron covariant propagation and combined with Monte-Carlo simulation.UrQMD model can describe hadron-hadron interaction and system evolution.In this thesis,we systematically analysed the various cumulants and cumulant ratios of event-by-event net-strangeness distributions at(?)=7.7,11.5,19.6,27,39,62.4 and 200 GeV with UrQMD model which concluding centrality dependence,rapidity dependence and energy dependence of cumulants and cumulant ratios from different particle classification.It concluded that the various cumulants and cumulant ratios have different centrality dependence and energy dependence behavior.Compared with net-kaon,we found the strange baryons make a great contribution to the fluctuations of net-strangeness.Although there is no QCD critical point involved in UrQMD model,this research can provide some reference to the study of QCD phase transition and critical point by observing the fluctuation of net-strangeness in heavy ion collisions.