Studies On QCD Fluctuation In Heavy Ion Collisions

Quantum chromodynamics(QCD)is a fundamental theory of strong interactions that successfully explains many physics phenomenon.QCD theory predicts that quarks confined inside hadrons at sufficiently high energies will be deconfined,forming a new form of matter-quark gluon plasma(QGP).In general,the QCD phase diagram is ex-pressed as a two-dimensional structure with baryon chemical potential ?B and tem-perature T.At high baryon chemical potential and low temperature range,the phase transition from hadron to quark gluon plasma is the first-order phase transition.As the decrease of baryon chemical potential and the increase of temperature,the phase transi-tion from hadron to quark gluon plasma is smooth transition.At the end of the first-order phase transition line,the starting point of the smooth transition zone,the theory predicts that there is a critical point(CP).At this critical point,the increase of the correlation length has a great impact on the experimental observation.Studies have shown that high-order fluctuations of conserved charges(net baryon number,net charge number and net strangeness number)are sensitive to the critical point,as shown by the high-order mo-ments of conserved charge and their product may be non-monotonic dependence on the collision energy.So,the high-order moments of the conserved charges are suggested as an observation of the critical point.Based on this theory,Brookhaven National Labora-tory's Relativistic Heavy Ion Collider(RHIC)completed the Beam Energy Scan(BES)experiment,the STAR collaboration group has already achieved some very interesting results,it shows that there is a critical point at the lower collision energy(7.7-19.6GeV).Using a multiphase transport model(AMPT),in this thesis,we study the effect of parton density fluctuations on high-order moments of net-proton,net-nucleon and net-baryon at low energy 7.7?11.5?19.6 and 27GeV.We analyzed the dependence of high-order moments such as net-proton,net-nucleon and net-baryon,as well as product on collision energy(?),rapidity and pT.We found that the effect of parton density fluctuation on high-order moments is not obvious,but it is found that high-order mo-ments and their product have an obvious dependence on the rapidity and pT.Moreover,we found that the high-order moments product ??2 of net-nucleon and net-baryon have a monotonous dependency on the collision energy in the AMPT.At the same time,we also used the quark dynamics coalescence model to study the effect of parton density fluctuation on the production,production ratios of ? and ? and their elliptic flow.The study found that under the quark dynamics coalescence model,the parton density fluctuation increase the production and the ratio of ? and ?,but have a small effect on the elliptic flow.By introducing parton density fluctuation into the AMPT model to simulate the process of QCD phase transition,we found that such a process could describe the ex-perimental results of the ? and ?.Our study provides a reference for the experiment to find QCD phase transition and critical point.