The Phenomenological Study Of Quark Combination Mechanism In Relativistic Heavy Ion Collisions

The production of hadrons in high energy collisions always involves a hadronization process, i.e. the process of the formation of observable hadrons out of quarks and gluons created in the collisions. It is a very important is-sue of strong interaction theory, i. e. quantum chromodynamics (QCD), at low energy scale, and meanwhile it is also, in relativistic heavy ion collision-s, a key bridge between final-state observables and the quark gluon plasma (QGP) produced in early stage of collisions. Because the QCD dynamics of the hadronization process are not yet fully understood, besides the limited calculations of lattice approach, at present it is mainly dealt with by the pa-rameterized fragmentation function and/or phenomenological models such as string fragmentation and parton combination.In recent years, the theoretic studies and experimental data at Brookhaven Relativistic Heavy Ion Collider (RHIC) suggest that, different from the jet-like structured few-parton system produced in elementary particle collisions like e+e- and pp, the system created in relativistic heavy ion collisions is a bulk matter with extremely high energy density and temperature, and exhibits the partonic degrees of freedom with relatively long duration at RHIC energies, i.e. the creation of QGP. Furthermore, the experimental data show hadron productions in the intermediate transverse momentum (pr) region exhibits a series of unique properties. In this pr region, the elliptic flows of baryons are much greater than those of the mesons and they are correlated to each other in terms of the constituent quark number scaling; the production of baryons is significantly enhanced relative to the meson production;and the baryons and the mesons also differ greatly in their nuclear modification factors RAA and Rcp in the same range. These phenomena can not be understood completely in the traditional parton fragmentation mechanism developed from the elementary particle collisions e+e- and pp, but can be naturally explained by the combination scenario of constituent quarks, which suggests the unique feature of hadron production in the environment of heavy ion collisions. The quark combination mechanism has successfully explained many experimental data of hadron production at RHIC and SPS, showing its indispensable role in describing relativistic heavy ion collisions. Meanwhile it also makes several existing issues or questions related to the mechanism become the hot spot of theoretical research, for example the condition and scope of its applicability (i.e. the universality) and the entropy issue in combination process, etc.In this paper, using a quark combination model of Shandong group (S-DQCM), we systematically investigate the production of thermal hadrons (i.e. hadrons with low and intermediate pr in the combination scenario) in heavy ion collisions from top RHIC to SPS energies, to test the applicability and uni-versality of quark combination mechanism as the effective QGP hadronization theory, and to extract the properties of QGP produced in heavy ion collisions. The hadron productions at SPS energies which have rich experimental data but relatively few studies are especially focused on. Distinguished from the inclu-sive formula of hadron production via quark combination in well-known quark recombination model and parton coalescence model, here we take advantage of the exclusive nature of SDQCM to systematically calculate the production of various light, strange mesons and baryons in heavy ion collisions. Especially we take the self-consistent explanation of the data of all hadrons as the test criteria of the applicability of quark combination mechanism. Taking advan-tage of the exclusive nature of SDQCM, we can extract a set of constituent quark distributions from the data of various identified hadrons which can self-consistently describe them in SDQCM, and analyze their features to obtain the properties of QGP in the early stage of collision evolution, e.g. collective flow and strangeness. Furthermore, utilizing this exclusive nature, we investigate the thermodynamic behaviors of the mechanism, e.g. entropy issue in quark combination process, as it is applied to describe the hadronization of partonic system with dramatic macroscopic characteristic in heavy ion collisions. The detailed contents and conclusions are as follows: (I) A systematic investigation of hadron productions in heavy ion colli-sions at SPS energies was made. The study was firstly focused on the Pb+Pb central collisions at top SPS ((?)=17.3 GeV). The yields, rapidity and transverse momentum distributions of various identified hadrons in most cen-tral collisions are calculated using SDQCM. The results are in agreement with the experimental data, and various baryon/meson ratios, e.g. p/π-, A/Ks0 andΩ/φas the function of pT, can be excellently reproduced. Using the thermal phenomenology of relativistic hydrodynamics, some properties of QGP space-time evolution, i.e. collective flows and strangeness, are obtained by analyzing the extracted quark distributions. It is found that (1) in partonic phase evolu-tion the collective flow of strange quarks formed finally is stronger than that of light quarks;(2) strangeness tends to be a saturated value about 0.44 in ultra-relativistic heavy ion collisions. Secondly, a energy scan of hadron productions via quark combination was made for all available collision energies of RHIC and SPS. Using SDQCM, we systematically calculate the rapidity distribu-tions of various identified hadrons in central nucleus nucleus collisions from top RHIC((?)=200,130,62.4 GeV) to SPS(Ebeam=158,80,40,30,20 AGeV) energies. Results show that, both at RHIC and SPS energies, given a set of quark rapidity distributions the model can self-consistently reproduce the rapidity distributions of various baryons and mesons. Particularly at 30 and 20 AGeV where the onset of deconfinement is suggested to happen, the mechanism can well describe the spectrum widths of various hadrons, which implies the possibly already existence of constituent quark degrees of freedom, but the properties of extracted quark distributions, i.e. spectrum width and strangeness, have novel behaviors, compared with higher collision energies. We further find that as the collision energy decreases to AGS 11.6 AGeV, the mod-el fails to simultaneously describe the rapidity distributions of pions, kaons and A which means there is no intrinsic correlation (at the constituent quark level) in their productions. These results suggest that the threshold collision energy of onset of deconfinement may be located around Ebeam=11.6-20 AGeV.(Ⅱ) On the basis of the correct description of single particle distribu- tions, the charge balance properties during the quark combination process are systematically investigated. The charge balance functions calculated by the model, either pT-integrated or different pT-cutting ones, all have the proper-ties of longitudinal boost invariance and rapidity scaling in the rapidity space. These properties are consistent with the observation of RHIC experiments.(Ⅲ) Two available methods, the Gibbs-Duhem relation and the entropy formula in terms of particle phase-space distributions, are used to study the entropy change in quark combination process. The entropy of the system extracted from the Gibbs-Duhem relation is found to increase in hadronization if the average temperature of the hadronic phase is lower than that of the quark phase. The increase of the entropy can also be confirmed from the entropy formula if the volume of the hadronic phase is larger than 2.5-3.0 times that of the quark phase. So whether the entropy increases or decreases during combination depends on the temperature before and after combination and on how much expansion the system undergoes during combination. The current study provides an example to shed light on the entropy issue in the quark combination model.Above results suggest that, both at RHIC and SPS energies where the QGP has been probably produced, the quark combination mechanism can self-consistently explain the production of various final-state hadrons, e.g. their yields and momentum distributions, which is suggestive of the universality of the mechanism as the effective hadronization theory of QGP.