The Rapidity Distributions Of Hadrons In Heavy Ion Collisions From RHIC To SPS Energies

The production of quark gluon plasma (QGP) and its properties are hot topics in relativistic heavy ion collisions. A huge number of possible QGP signals were proposed and measured by experiments, and many unexpected novel phenomena were observed at RHIC and SPS. These experimental data greatly contribute to the identification of QGP and the understanding of its properties and hadronization mechanism from different aspects. Especially, there are a class of phenomena that are of particular interest, i.e. the big difference between meson and baryon production in the intermediate PT range. The experimental data from RHIC have shown that, in this range, the elliptic flow of baryons is much greater than that of mesons, and correlated with that of mesons by the constituent quark number scaling; furthermore, the production of baryons significantly enhances relative to the meson production, i.e. the well-known high baryon/meson ratio; and baryons and mesons also differ greatly in their nuclear modification factors RAA and Rcp. This is a unique characteristic of relativistic heavy ion collisions which reveals the mechanism of hadron production.In quark (re-)combination/coalescence scenario, hadrons are combined from quarks and antiquarks, that is, a quark-antiquark pair merges into a meson and three quarks into a baryon. The difference of production between baryon and meson mainly results from their different constituent quark numbers. It is shown that such a quark number counting can successfully explain the experimentally observed difference between baryon and meson at RHIC energies. Considering the flavor difference of light and strange quarks, the mechanism can self-consistently reproduce the data of various light, strange mesons and baryons. Of course, such production correlation between meson and baryon via quark number counting is based on the existence of constituent quark degrees of freedom in relativistic heavy ion collisions, which is closely related to the creation of partonic bulk matter.These highlights at RHIC stated above are mainly of hadron production in transverse direction where quark recombination scenario mostly flashes. In fact, the longitudinal rapidity distributions of hadrons are also good tools of testing the scenario. The rapidity density of hadrons is mainly contributed from the hadrons in low PT region. In this region quarks and antiquarks are more abundant in phase space, compared with those in the intermediate PT region, so the combination happens more easily and naturally and should dominate the observed rapidity distributions of hadrons in relativistic heavy ion collisions and can be tested by the data. Compared with the direct (vector) summation of PT in transverse combination, the change of rapidity during combination may be not so explicit but the rapidity distributions of various hadrons formed via quark recombination are definitely distinguished. The rapidity distribution of a specific hadron in the mechanism is the convolution of the rapidity spectra of their constituent quarks and combination probability function. Since the rapidity distributions for different flavors of quarks are different, in particular the difference between newborn quarks and net-quarks coming from colliding nuclei, the shapes of rapidity spectra of hadrons with different quark components are different and correlated with each other. Therefore, the applicability of the mechanism can be tested by identifying this quark-level correlation underlain in the experimental data of hadronic rapidity distributions, or equivalently by studying wether the data of various hadron species can be simultaneously explained by a set of quark rapidity distributions. In this paper, we use a quark combination model to systematically investigate the rapidity distributions of various identified hadrons in central nucleus nucleus collisions in a broad collision energy region (from top RHIC to SPS).The work contains two aspects as follows:(I) Combining the past work, we systematically study the rapidity spectra of constituent quarks in central A+A collisions from RHIC energies (?)=200,62.4GeV to SPS energies Ebeam=158,80,40,30,20AGeV. There are obvious changes in such a broad energy interval, i.e. The dual hump shape of rapidity spectrum of net quarks are disappearing with the collision energies descent, the Gaussian width of the rapidity distribution of newborn light quarks reduces gradually with the decreasing energies. Besides, we notice another interesting phenomenon that the Gaus-sian width of newborn ud quarks is larger than that of s quarks at 20 AGeV while at other energies are contrary.(II) In this section, we systematically calculate the rapidity distributions of identified hadrons in central A+A collisions from RHIC (?)=62 AGeV to SPS Ebeam=80,40,30,20AGeV under the framework of the quark combination. It is shown that the quark combination model can describe the experimental data well in such a broad energy interval.