Hadron Yield And Its Associated Energy Dependence

According to lattice QCD, people predict that at extremely high temperature or high energy density, the confined hadronic matter will undergo a phase transition to a new state of deconfined matter—quark gluon plasma (QGP). Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Lab provides an environment to create QGP. A variety of experimental phenomena at RHIC have indicated that QGP has been probably produced. The experimental data about Pb+Pb collisions at top Super Proton Synchrotron (SPS) energy from NA49 Collaboration show similar properties to those of RHIC energies. This implies that QGP has been produced at top SPS, if it has been produced at RHIC energies. Has QGP been produced in A+A collisions at lower SPS energies? Which energy point does QGP first appear at? The Beam Energy Scan programme of NA49 experiment at CERN-SPS has suggested a preliminary answer—around 30AGeV. The ongoing Beam Energy Scan programme of STAR Collaboration at Brookhaven National Lab provides an opportunity to study it in more detail.Once the deconfined hot and dense quark matter is produced in heavy ion collisions, the observables of various thermal hadrons after hadronization, e.g. yields and momentum spectra etc, have some correlations originated from early quark degrees of freedom. One of the most typical examples is the elliptic flow v2 of hadrons measured at RHIC energies. As both v2 and transverse momentum PT are divided by the constituent quark number of hadron, the rescaled v2 of various baryons and mesons, which are just that of constituent quarks, almost coincide with each other in the intermediate PT range. If the hot and dense quark matter is hadronized by quark combination, as is commonly accepted, these correlations of hadrons can be beautifully explained. In quark combination scenario, quarks and anti-quarks are available in unbound state before hadronization and they can coalesce freely into various hadrons, and thereby these correlations from early quark degrees of freedom among different hadron species are naturally formed. On the other hand, if the deconfined quark matter is not produced at all in collisions, there is no free quarks and anti-quarks (much less their subsequent combination) and these so-called "quark-level" correlations of hadrons maybe disappear or contort. Therefore, we can study whether the deconfinement is achieved by investigating these correlations among various hadrons produced in heavy ion collisions. The work contains two aspects about whether there is deconfined quark matter and the hadronization mechanism in heavy ion collisions as follows: (Ⅰ)We study the yield densities of various hadrons at different energies. Hadron yield is one of the most fundamental and significant observables from which one can obtain a lot of important information on the hot nuclear matter produced at the early stage of relativistic heavy ion collisions. We systematically study the yield densities of various hadrons at mid-rapidity from RHIC energies SNN^1/2= 200,130,62.4GeV to SPS energies Ebeam=158,80,40,30,20AGeV in heavy ion collisions. It is shown that the quark combination model can describe the experimental data well from Ebeam=30AGeV to SNN^1/2=200GeV and this energy region is just the region claimed by NA49 Collaboration where QGP maybe has been produced. At20AGeV, the quark combination model fails. This suggests that the constituent quark degrees of freedom do not represent a decisive factor in thermal hadron production. Furthermore, we predict hadron yields as well as particle ratios at mid-rapidity in the most central Pb+Pb collisions at SNN^1/2=5.5TeV. (Ⅱ)We also investigate the hadron yield correlations at different energies. In this work, we lefine two correlation quantities A=(ΛK-p)/ΛK+p and B=(ΛK-Ξ+)/(ΛK+Ξ-), sensitive to quark degrees of freedom. The values of A and B for directly produced hadrons are equal to 1.0 in the framework of quark combination, independent of special models. The deviation of A and B from 1.0 or not can be regarded as a possible signal of deconfinement in heavy ion collisions. Following the experimental data at different collision energies, we evaluate the values of A and B. In order to explore the decay effect, we use the quark combination model to compute the values of A and B for the directly produced hadrons, and find that as the collision energy is greater than or equal to 30AGeV the values of A and B are nearly equal to 1.0, while at 20AGeV, it deviates from 1.0. This supports the conclusion to some degree drown by NA49 Collaboration that the onset of deconfinement may be located around 30 AGeV.