Top Sps Energy Hadrons Produced In Pb + Pb Collisions

Lattice QCD predicts that at extremely high temperature and density, the confined hadronic matter will undergo a phase transition to a new state of matter called quark gluon plasma (QGP). The relativistic heavy ion collisions can provide the condition to create this deconfined partonic matter. In general, two approaches are used to study the properties of the deconfined hot and dense quark matter produced in AA collisions. One is studying the high p_T hadrons from initial hard jets, in which one can recur to the perturbative QCD to a certain degree. The other is investigating the properties of thermal hadrons frozen out from the hot and dense quark matter. For the latter, the hadronization of the hot and dense quark matter (a typical non-perturbative process) is of great significance. Only through a reliable hadronization mechanism, can we reversely obtain various information of QGP properties from the final state hadrons measured experimentally. The abundant experimental data and phenomenological studies at RHIC energies suggest that quark combination mechanism is one of the most hopeful candidates. The two most noticeable results are the successful explanation of the high baryon/meson ratios and the constituent quark number scaling of the hadronic elliptic flow in the intermediate transverse momentum range, which can not be understood at all in the partonic fragmentation picture. Recently, the NA49 Collaboration have measured the elliptic flow of identified hadrons at top SPS energy, and found that the quark number scaling of elliptic flow was shown to hold also. It immediately gives us an inspiration of the applicability for the quark combination at top SPS energy. On the other hand, the NA49 collaboration have found three interesting phenomena around 30A GeV, i.e. the steepening of the energy dependence for pion multiplicity, a maximum in the energy dependence of strangeness to pion ratio and a characteristic plateau of the effective temperature for kaon production. These phenomena are indicative of the deconfinement at low SPS energies. One can estimate via Bjorken method that the primordial spatial energy density in Pb+Pb collisions at top SPS energy is about 3.0 GeV/fm³, exceeding the critical energy density (about 1 GeV /fm³) predicted by Lattice QCD. Therefore, the deconfined hot and dense quark matter has been probably created, and we can extend the quark combination mechanism to SPS energies.Considering the rich experimental data at top SPS energy, in this paper, we apply the quark combination model to systematically study the yields, rapidity and transverse momentumdistributions of various hadrons in most central Pb+Pb collisions at (S_NN)^1/2=17.3GeV. On one hand, one tests the applicability of the quark combination mechanism at this collision energy. On the other hand, the parton momentum distributions at hadronization, which carry the information on the evolution of the hot and dense quark mater, are extracted from the final hadrons at top SPS energy and compared with those at RHIC energies. We concentrate the comparison on two properties related to strange hadron production. One is the difference in collective flow between light and strange quarks, which occurs at RHIC energies. The other is the strangeness enhancement, a significant property of QGP.The work contains two aspects as follows:(Ⅰ)We use the quark combination model to study systematically the momentum spectra of final hadrons at top SPS energy. The transparency of top SPS energy is weaker than that of top RHIC energy, so amount of net quarks from colliding nuclei are stopped in the collision area, and their longitudinal evolution is different from the newborn quarks. Therefore, we distinguish the momentum spectrum of net quarks from that of newborn quarks, the momentum distributions ofπ^±, k^±, p(?),Λ(?), k_s⁰, (?) can be naturally described. We compute the baryon/meson ratios and find that the quark combination model can describe it. (Ⅱ) We also investigate the longitudinal and transverse collective flows and strangeness of the hot and dense quark matter produced in nucleus-nucleus collisions at top SPS energy . We find that the collective flow of strange quarks is stronger than light quarks, which is compatible with that at RHIC energies. We abstract the strangeness from the nucleus-nucleus collisions at (S_NN)^1/2 = 17.3GeV,62.4GeV,130GeV,200GeV and find the strangeness exhibits an obvious saturation phenomenon.