The Charged Particle Pseudorapidity Distribution

Since the operation of Heavy Ion Collider(RHIC)at Brookhaven National Laboratory(BNL) in 2000,many interesting observations which indicate the existence of quark gluon plasma(QGP) in high energy nucleus-nucleus collisions have been obtained.As the deconfined quarks and gluons can not be detected directly,we have to study the properties of QGP indirectly through the properties of final hadrons with the hadronization model.Currently,there are two popular mechanisms of hadronization:one is parton fragmentation mechanism,and the other is quark combination mechanism.Quark combination model has successfully described many experimental observations,such as rapidity and momentum distributions of identified particles, the scaling of the elliptic flow,the ratio of baryon to meson,flavor dependence of the nuclear modification factor and so on.,which can not be explained by fragmentation mechanism. Nowadays,the quark combination model has been accepted widely.In this paper,within the shandong quark combination model(SDQCM),we study the energy,centrality and system size dependence of pseudorapidity distribution.A large number of charged particles are produced in nucleus-nucleus collisions at RHIC.The rapidity,energy,centrality and system size dependence of pseudorapidity density dN/dηis interesting physical quantity,which shows the evolution of initial system,the properties of dynamical and stopping.In paper Phys.Rec.C.2007,75:034904,employing SDQCM,the authors have successfully described the pseudorapidity distribution with Landau hydrodynamical model in Au+Au collisions at different energies and centralities.However,their results are slightly lower than the data in forward rapidity range.The main reason may be that they have not considered the leading particle effect.In this paper,we consider the contribution of leading particles,and apply three fireballs to describe the system before hadronization.The big central fireball which contains the main part of collision energy,controls the rough shape of charged particle pseudorapidity distribution(width and height).The two small fireballs in forward rapidity range carry the information of leading particles.We describe the rapidity distribution of quarks in mid-fireball and forward rapidity range by Landau relativistic hydrodynamic model and small Gauss distribution,respectively.The main contents of this thesis are as follows:Firstly,we study the charged-particle pseudorapidity distribution in Au+Au and Cu+Cu collisions for different centralities at(S_NN)^1/2=19.6,62.4,130 and 200 GeV.We find that:(ⅰ)the contribution of charged particles produced in forward rapidity range decreases with the increase of the centrality,although the number of charged particles produced in two forward small fireballs increases with the increase of centrality.(ⅱ)The ratio N_ch(T/P)/N_chwill decrease with the increase of the energies for the same centrality.