Fig. 1-5 Rapidity distributions of the numbers of negative hadrons (), and net protons obtained by subtracting the number of antiprotons () from that of protons, produced in the collision of sulfur nuclei with momentum of 6,400 GeV/c, where c is the light velocity. Simbols, and , represent the observed data by the NA35 group of CERN while the histograms give the result of PHCascade calculation. The symbol b is the impact parameter giving the deviation from the head-on collision of two sulfurs.

Fig. 1-6 Comparison of the results of the simulation for the rapidity distribution of the net-proton number produced in the S-S collision at the accelerated energy designed for the heavy ion accelerator RHIC at BNL.

In order to observe the behavior of quarks and gluons, constituents of hadrons such as protons, anti-protons, neutrons, etc., a high-temperature-high-density state is required. This is produced by giving ultra-high energies to the nucleus. For this purpose, processes induced by heavy ions colliding against heavy nuclei with more than a few hundred GeV are studied. Collision events proceed with time to hadron formation after the local thermal equilibrium has been attained in the quark-gluon plasma via preequilibrium states. Though several ultra-high energy experiments have been attempted so far, any definite evidence has not yet been obtained. A Parton Hadron cascade model (PHCascade) was introduced to carry out a simulation for the collision processes between sulfur nuclei. Here, the parton signifies a quark or gluon which behaves as a free particle. Figure 1-5 demonstrates comparison between the calculated rapidity distributions of the produced negative hadrons and the net number of protons, and the experimental results at CERN. Here, rapidity gives a measure of the transferred momentum to a different incident direction. The result shows that the calculation reproduces the observed data very well and, therefore, the PHCascade describes the phenomenon satisfactorily. Figure 1-6 gives the results of model calculations of the rapidity distribution of the net proton number produced in the S-S collision at the much higher energy, designed for RHIC (Relativistic Heavy Ion Collider) at BNL; calculations using three models, two typical models, HIJING and VNI, and the presently developed PHCascade. It should be noted that HIJING and VNI give zero rapidity distributions in the central zone (-2 < y < 2), while PHCascade predicts finite net proton densities in the central almost zone, that is, the formation of plasma with finite baryon number densities.

Reference Y. Nara, A Parton-Hadron Cascade Approach in High-Energy Nuclear Collisions, Nucl. Phys., A638, 555c (1998).

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