Research On The New State Of Matter And Hadronization On High Energy Nucleus-Nucleus Collisions

At extremely high energy densities, Quantum Chromodynamics (QCD) predicts a new form of matter, consisting of an extended volume of interacting quarks, antiquarks and gluons. This is quark-gluon plasma (QGP). The relativistic heavy ion collision is a significant experimental tool for deep understanding of this new form of matter, through which scientists expect to realize the transition from nuclear matter to quark matter. The research of final observables in nucleus-nucleus collision, such as:the particle distribution, correlation and fluctuation, etc, is greatly important for understanding the evolution of the high energy heavy ion collision and the particle production mechanisms. This dissertation mainly focuss on the particle production mechanisms and properties of particle distributions, with the data from the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory.In the first part, the status of heavy ion collision is briefly reviewed, both in experimental and theoretical aspects. Then some important definitions and phenomenological models are introduced, including the Glauber model, Recombination/Coalescence (ReCo) Model by the Oregon group and ALgebraic COalescence Rehadronization model (ALCOR).At very high temperature, due to the asymptotic freedom, the interactions among the partons are so weak that one could approximately treat the system as an ideal gas of quarks and gluons. However, near the critical temperature Tc, it is very difficult to solve QCD by analytical methods, such as the perturbative theory, because the coupling constant is not small enough. Lattice QCD can be used to calculate, from the first principle, the properties of simple systems. But this method is time consuming and needs big computing capacity. Hence, a self-consistent quasiparticle model is presented to investigate the thermal properties of quark-gluon plasma above the critical temperature. All effects of the interactions among the partons are contained in the thermal mass of partons, so the system may be treated as an ideal gas of the massive quarks and gluons. From the statistical mechanics, we can calculate the thermodynamical quantities without any inconsistency, and then obtain a good fit to the lattice QCD data.We also investigate the sensitivity of the hadron yields on the use of the different quark and hadron wave functions. The results show that the meson production rates strongly depend on the gluon mass at fix temperature (here T = 180 MeV). Considering the temperature dependence, the rates are very much insensitive on this parameter at fix gluon mass (mg= 800 MeV). With the ratioΦ/K*=0.60±0.15 measured at RHIC in central Au+Au collisions at (?)= 200 GeV, one could get that the hadron rations are insensitive on the wave function setups. These results prove most strikingly why the coalescence models yield very good agreement during data reconstruction. Besides, the problem of forward production of hadrons in heavy-ion collision at RHIC is revisited with modification of the theoretical treatment on the one hand and with the use of new data on the other. The basic formalism for hadronization remains the same as before, namely, recombination, but the details of momentum degradation and quark regeneration are improved. Recent data on the p/πand (?)/p ratios are used to constrain the value of the degradation parameter. The transverse momentum pT spectrum of the average charged particles is well reproduced. A prediction on the PT dependence of the p/p ratio atη= 3.2 is made.With the careful consideration of the initial geometry, We have given a unified description of all the azimuthal dependencies of all observables onπproduction at low pT (pT＜2 GeV/c) in heavy-ion collisions. They are:the nuclear modification factor RAA(φ,Np), elliptic flow coefficient v2(Np) and ridge yield YR(φs, Np) as functions of Np, the number of participants. The main physics input is that the semihard scattering near the surface drives the azimuthal anisotropy on the one hand and the production of ridge particles on the other with or without trigger. The geometrical factor, S(φ, b), that makes precise the bridge between the two aspects of the problem follows from a study of the correlation between theφdirections of the trigger and ridge particles. The main understanding achieved in this picture is that the single-particle distribution dNAAπ/pTdpTdφhas two components:one is theφ-independent bulk B(pT,Np), different from the conventional bulk that isφ-dependent, and the other is the ridge component R(pT,φ,Np) that carries all theφdependence.Finally, With the experimental data from STAR, PHENIX and BRAHMS programs on the centrality, rapidity and energy of transverse momentum pT spectrum in Au+Au and d+Au colli-sions, we show that there exists scaling distributions for pion, proton and antiproton. The difference between the scaling functions for protons and antiprotons is quite small, but they differ a lot from that of pions. From the scaling functionsΨ(u), one can see that the only parameter characterizing the normalized distribution is the average transverse momentum which depends on the cen-trality, rapidity and the colliding energy and system. Once is known, both the soft part with low pT and the hard part with high pT are determined byΨ(u). Using the obtained scaling func-tions of pions and protons, we could describe the charged particle pT spectra at different charged particle multiplicities in p+p collisions as a superposition of those from pion and proton. Different from momentum, kinetic energy is a scalar and is directly associated with the temperature of the hot medium created in the collisions. For different species of particles, the same momentum corre-sponds to different kinetic energy because of mass effect. Thus the distributions of kinetic energy of particles produced in ultra-relativistic heavy ion collisions are more effective in revealing the thermal properties of the system. So we pursue to investigate the scaling properties of transverse kinetic energy ET distributions of pions and protons, and find the agreement of the scaling ET distribution is better at low and high region. Besides, in the two frameworks:string fragmentation, cluster formation and decay, the universal transverse energy distributions for pion and proton can be described separately but not simultaneously. This fact indicates that they are not the universal mechanisms for the production of final state pions, protons, and other particles in high energy collisions. Obviously more detailed studies, both theoretically and experimentally, are needed.