The Study Of Universality For Quark Combination Mechanism In QGP Hadronization Process

People began to study the properties of nuclear matter under extreme conditions from both theory and experiment in the early 1970s.From then on the relativistic heavy ion collisions have become the frontier of modern physics.Since the startup of Relativistic Heavy Ion Collider(RHIC) at BNL in America,lots of attractive data are published.And there are many unexpected 'spotlights',e.g.,the baryon enhancement in the intermediate p_T range,the nuclear modification factor R_CP separating into two groups of baryons and mesons,the valence Quark Number Scaling of hadronic elliptic flows,and the disappearing of the back-to-back azimuthal correlation of high p_T bi-hadrons.These observations imply strongly that the partons are created in RHIC reactions.The most persuasive proofs are the elliptic-flow and R_CP behaviors forφmeson,which has the same mass but a different valence quark content with proton.However,φmeson behaves like other, lighter mesons,not like proton.In short,experiments at RHIC suggest the production of the hot dense parton matter,and the combination,instead of the fragmentation,is the main hadronization mechanism at least in the intermediate p_T range.Then,is the combination valid at low and high p_T? And is it the universal hadronization mechanism? They are the main issues discussed in the thesis.Hadronization can not be solved by perturbative QCD.Besides the approach of Lattice QCD,there are two kinds of methods widely used:the fragmentation and combination models.The combination models in RHIC collisions are mainly Coalescence,Recombination,and our Quark Combination Model(QCM).In the early 1980s,based on the coalescence picture,Q. B.Xie et al.proposed systematically the quark production rules and the quark combination rules.Then Q.Wang,Z.G.Si et al.extended them into multi-patton system,and successively reproduced many data in pp((?)) reactions and e⁺e^- annihilations.The quark production rules calculate the averaged number of new born quarks for the q(?) color singlet system with the energy S.All quarks combine into hadrons by the quark combination rules which satisfy the fundamental requirements of "near correlation in rapidity" and the QCD.The demand of "near correlation in rapidity" determines the rule of 'meson production prior to baryon' in the hadronization process.In addition,the combination picture can explain naturally the "spin suppression" and the high yield of the flavor singlet baryon in the baryon production. Recently,F.L.Shao et al.have developed the QCM and applied it in the RHIC reactions.At first,the quark production rules are extended and with it the total number of quarks at various energies and centralities in RHIC collisions can be obtained.While the quark combination rules,based on the primary "near correlation in rapidity",are added with the combination condition of equal or similar p_T of collinear quarks.As QCM satisfies the unitarity and does not need the explicit combination functions of various hadrons,it can systematically describe the properties of all SU(3) or SU(4) hadrons.It is the advantage which other combination models do not have.By using the extended QCM,we firstly study the hadronic yields,ratios and their spectra in RHIC reactions.After determining some parameters of QCM,we obtain the results consistent with the data very well,and reproduce the 'abnormal' baryon enhancement in the intermediate p_T range.In high energy nucleus-nucleus collisions,Hwa and Yang find out the universal scaling ofÏ€⁰ spectra at various energies and centralities,which is proved by QCM.Our study suggests that,same as in the intermediate p_T range,the combination mechanism is available at both low and high p_T on reproducing the hadronic spectra and yields.Secondly,we study the hadronic elliptic flows and their mass hierarchy with QCM.The elliptic flowν₂ is one of the most important observables in relativistic heavy ion collisions.It carries lots of information on the initial state of the dense matter and on the hadronization process.One of the high-lights in the first few years' run of RHIC is the discovery of quark number scaling(QNS) ofν₂ for different hadrons.It is one of the main reasons which leads people to propose that quark combination is the dominant hadronization mechanism of QGP.However,the later more abundant data at low p_T show that QNS seems not to be valid.Instead,there is an obvious structure, referred as the mass hierarchy of elliptic flows.Does it suggest the combination is not available at low p_T? Through the systematical study of the factors that can influence the mass hierarchy,such as the resonance decay,the flavor dependence of the quark elliptic flow and the combination of(anti)quarks with slightly different transverse momenta,we find that combination can exactly explain the hierarchy.The decay effect leads to an obvious structure ofν₂(p_T) for hadrons at low p_T.However,they are not enough to reproduce the data for all hadrons.The mass effect of constituent quarks results in the splittings ofν₂(p_T) curves for prompt hadrons according to their flavor contents and even larger splittings for final state hadrons.A reasonable agreement between the calculations and data is achieved for all the hadrons except forÏ€^±.Unequal p_T combination leads also to the deviation from QNS,which can be in different direction depending on the shape ofν_2,q(p_T) of quarks. We also find that the three effects all lead to violation of the QNS.The first two manifest themselves mainly at low p_T,while the violation from the unequal p_T combination is more apparent in intermediate p_T range.By taking all the three effects into account,we have reproduced the hierarchy ofν₂ for hadrons includingÏ€^±in the low PT region.Further prediction of criteria is given to judge whether unequal p_T combination contributes significantly in hadronization process.Another point should be emphasized that the combination mechanism is universal at low and intermediate p_T in describing the hadronic elliptic flows.Within QCM,we also study the distributions of(pseudo)rapidity densities for charged particles at various centralities and energies in Au+Au collisions.Using Landau relativistic hydrodynamics to deal with the evolution of parton matter,a Gaussian-type rapidity distribution for constituent quarks before hadronization is obtained,and the pseudorapidity(η) densities for charged particles at 130,200 GeV at various centralities in fullη range are well reproduced.We further study the energy dependences of the pseudorapidity densities,and find that the shape ofηdistribution is mainly determined by the energy and the sound velocity,and the rapidity width of quarks only influences the forward(back)ηof hadrons.Interestingly,except for 19.6 GeV,the sound velocity of QGP is the same at 62.4,130,and 200 GeV,which indicates a kind of universality for the quark matter produced in these energies.However,the hot dense matter at 19.6 GeV do not possess the universality.At last,we extend the QCM to including the broken SU(4) flavor symmetry of quarks,and study quantitatively the charm production in relativistic heavy ion collisions.Charm production in high energy heavy ion collisions is one of the hot topics of both theory and experiment.PHENIX and STAR collaborations have made many measurements on it.The binary scaling of the total charm cross sectionσ_c(?) has been observed by them.However,they give quite different values of the binary scaled charm cross sectionσ_c(?)^NN.There are two important ratios used for obtainingσ_c(?)^NN in the experiments.One used by PHENIX is R_e/c(?),the inclusive branching ratio from c(?) to e^-(e⁺) in AA reactions.Another used by STAR is R_D⁰/c(?),the inclusive branching ratio to D⁰((?)⁰).The two ratios and the total branching ratio from c(?) to muons R_μ/c(?) are all relative to the charm hadron ratios.The charm cross section is one of the most important issues of charm physics in heavy ion collisions.The accurateσ_c(?)^NN measurement depends on the accurate ratios R_e/c(?)(R_μ/c(?)) and R_D⁰/c(?),so it is an important issue of charm physics to determine R_e/c(?),R_D⁰/c(?),R_μ/c(?) and the charm hadron ratios in AA collisions.The baryon enhancement and the strangeness enhancement in AA reactions will result in the different charm hadron ratios and total branching ratios from those in the pp((?)) reactions or e⁺e^- annihilations. However,it is now difficult to measure them directly in experiments, and these ratios cannot be obtained through theory calculations model-independently.Then one must ask,how large are the corrections toσ_c(?) from the two ratios in Au+Au collisions,and are the corrections able to account for the discrepancy ofσ_c(?)^NN measured by the two collaborations?Within QCM we calculate at 200 GeV the p_T dependencies of the charm baryon-to-meson ratios,such as A_c+(?)_c/D⁰(?)⁰,A_c+(?)_c/D⁺+D^-,and A_c+(?)_c/D_s⁺+(?)_s^-.It is clear to see that the charm baryon enhancement in intermediate p_T range is very prominent, which is similar to that of the p/Ï€ratio.Comparing with the data of p/Ï€⁺ in the same Au+Au collisions,one can see that all peaks broaden and shift to the right because essentially the spectrum of the c(?) is much harder than that of the u(d) quark.We discuss the R_AA ordering with strangeness content for charm hadrons in heavy ion collisions,and point out it is also a powerflfl signal for proving the combination hadronization mechanism for open charm hadrons.We also calculate the midrapidity densities of the single-charm hadrons and their ratios at 200 GeV.These ratios in AA collisions, including the key ratios R_e/c(?) and R_μ/c(?),are apparently different from those in pp((?)) reactions and e⁺e^- annihilations because of the charm baryon enhancement and the strangeness enhancement.With the newest branching ratios from PDG,the modified charm hadron ratios lead to a～17%increase of the central value ofσ_c(?)^NN measured by PHENIX.However,this correction is not enough to account for the discrepancy ofσ_c(?)^NN between STAR and PHENIX.Considering the uncertainties of some parameters,we systematically explore the charm-ratio dependencies on various parameters.We further investigate the centrality dependencies of these ratios at 200 GeV in Au+Au collisions,and find that the ratios of single-charm hadrons and the three key ratios are all independent of the centrality as long as the binary scaling ofσ_c(?) is held and those parameters are irrelative to the centrality.Assuming the combination hadronization mechanism is universal in relativistic heavy ion collisions,we further investigate the energy dependencies of these ratios,and reveal that they are mainly dependent on strangeness suppression factor and the net baryon number but not on theσ_c(?).The predictions of charm hadron ratios at S_NN^1/2= 62.4,130,and 200 GeV for the upgrade of RHIC and 5.5 TeV for LHC are given.It is the first time that in combination mechanism,the extensive charm hadron ratios and the total branching ratios R_e/c(?),R_D⁰/c(?),R_μ/c(?) in AA collisions are predicted at RHIC and LHC.These ratios are important for the precise measurement ofσ_c(?)^NN in the future,and the combination hadronization mechanism for charm hadrons can be tested at RHIC and LHC.In summary,we study some aspects of RHIC reactions within QCM,and theoretical results are consistent with the data well.Some QCM predictions are also given.Especially,our work proves that not only in the intermediate but also in the low and high p_T range,the combination mechanism can describe the hadron properties very well.This supports powerfully the opinion that the quark combination is a universal hadronization mechanism.