Particle Production and Nuclear Fragmentation in S32 O16 Collisions AT 14.6. 60 and 200A GeV Energies

Abstract

(9.1) Statement of the research problem: Though the non-perturbative feature of Quantum Chromodynamics (QCD) such as confinement have not yet been rigorously established, it is widely believed on the basis of lattice Monte Carlo that either at high temperature or high densities normal hadronic matter undergoes a phase transition to a new confined state of matter .the Quark Gluon Plasma (QGP) formed by the basic constituents of the hadrons, i.e the quarks and gluons (1). Estimates of the critical temperature and energy density are Tc — 200 MeV, p = 2-3 GeV / fm3 (2). Study of this phase transition is important to understand the evolution of the universe. Generally it is admitted that the phase transition occurred effectively at critical temperature Te — 100 MeV at QCD time scale (3). It is possible that, if the transition were delayed, for this critical temperature a high degree of cooling is obtained which may allow the possible formation of large quark stars (4-6). Study of the QCD phase transition is therefore important from the astrophysical point of view also. The relativistic heavy ion collisions provide the best way to understand the properties of the hot and dense matter produced in the laboratory (7). The purpose is to reach ultimately the condition under which hadronic matter is transformed into QGP, and this forms the basis of experimental heavy ion program around the world. An impressive program of investigations has been completed at BNL (AGS) with 14.6A GeV Si28..beam and at CERN (SPS) as well as through emulsion collaborations using S32 and 016 beams at 60A GeV and 200A GeV (8,9). Temperature of about 200 MeV and energy density of about 3 GeV / fm3 are already reached in S32 collisions at the incident energy of 200 A GeV. The crude estimate of p —1.3 GeV / fm3 at BNL and p 3 GeV / fm3 at CERN indicate that the appropriate conditions are possibly met for the QGP formation at CERN (10), but the question arises that how can we characterize the phases before and after transition ?. This question is related to that of finding unambiguous signals of the QGP. More recently experiments have been performed with 158A GeV Pb ion beam through NA50, NA97 and WA98 collaborations at CERN (11). The data collected over the last 10 years at CERN and BNL, clearly indicate that at the highest energies achieved so far, nuclear collisions • deviate substantially from a picture based on a mere superposition of independent. nucleon-nucleon collisions, i.e collective behaviour is seen. While a coherent picture of the collision dynamics is emerging, finding unambiguous signal of QGP formation is an open question. In the most recent experiments at BNL (Au beam) and CERN (Pb beam) there has been restrictions to sirals of QGP (12). Among the observed signals the J / iii suppression, the excessive production of strange and multistrange baryons and enhanced emission of lepton pair have attracted more attention, but explanation other than the QGP formation has been given. Thus at the moments it can only be conceived that we do seem to have produced very dense matter. But to decide whether it is "still " hadronic matter or a QGP, we need more experimental information. Tremendous efforts have been made on theoretical front. Taking microscopic models for the nuclear reaction in the SPS energy domain, detail comparison with the new high precision data reveal that none of the existing hadronic cascade models really work correctly at SPS energies. The string models like Dual Parton Model (DPM) and Lund Model (13,14) seem to correctly describe main part of the experimental data. Statistical models (15,16) have been used to describe the statistical production systematic in high energy nuclear collision of the hadron multiplicities. But there is no theory available to describe the background in heavy ion collision necessary to amplify the phase transition signals, because we do not know how to apply the QCD to soft processes (small momentum transfer) which are the bulk of the events taking place in heavy ion collisions. In order to understand the background on which the proposed signals from the QGP can be searched for, questions related to the normal reaction mechanism have to be answered. The dynamic of nuclear reaction is complicated and at present allows at best for only semi- quantitative predictions. We therefore need to learn the detail of how the heavy ion collisions work before we can draw any general conclusion about properties of hadronic matter. Progress in the field is therefore largely conditioned by progress in experiments. Thus it is needed to investigate the main features of reactions experimentally and make the model comparison to understand the reaction dynamic before looking for definite signals. From a careful look at the available experimental information it may be concluded that certain important proposed aspects like variation of intermittency exponent with • the mean number of collisions (17,18) have been overlooked in the relativistic high energy collisions or the data is very small. In order to understand particle production in the nuclear environment, the behaviour of leading particles, production of secondaries in different rapidity regions and cascading of secodaries in the nuclear medium have to be investigated.Thus measurements on multiplicity, rapidity density and nuclear fragmentation are carefully required. The factorial moments were proposed (19,20) to study the intermittency (fluctuations of different sizes) in multiparticle production at high energies. It has been found that (21-23) that intermittency at different high energies. The intermittent behavior of multiparticle production may be related to a second order phase transition from a QGP to normal hardonic matter or to the random cascading process. The anomalous fractal dimensions, with the rank ‘q’ of the moment, were suggested (24) to be a good tool to differentiate between the second phase transition and random cascading process in the heavy ion reactions. This important effect, Though somewhat studied (23,25) needs to be investigated further in much greater detail for an understanding of the phase transition. These are the main problems which need to be addressed and form the basis for our present investigation using O16 and S32 ions at 14.6,60 and 200A GeV energies.