Study on Quark-Gluon-Plasma in the High Energy Heavy Ion Collision

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Final goal of this research project is to detect and investigate a Quark-Gluon Plasma (QGP), a new phase of high-energy and high-density nuclear matter which is expected to be formed by the high energy heavy ion collisions. As a first step of this study of new phase of nuclear matter, we started from the measurements of the inclusive spectra of pions, kaons, and protons produced by the heavy ion collisions at the incident energy of 14.5 GeV/nucleon at BNL-AGS.The measured transverse kinetic energy distributions of hadrons are well described by an exponential form in the measured region. The inverse slope (slope parameter) for protons is found to be larger than that of pions at the rapidity region from 1.1 to 1.5 in Si-induced reactions, whereas the slope parameter depends neither on emitted particle species nor on mass number of target nuclei.The yield ratio of kaons to pions is especially interesting because several theories predict this ratio will be a good signal of the QGP formation. In our measurements this ratio was found to be 14.9 % for positive kaons and pions, and 4.7 % for negative kaons and pions in Si-Au collisions. After the corrections for the spectator effects, the above ratios in the participant region become 8% for positive particles and 3% for negative particles. Although these ratios are by a factor of 1.5 larger than in p-p collisions, they are not as large as the predicted ratios from the QGP. However, the continuous study of this effect will be necessary.Besides these hadron measurements, we did the computer simulations to study the possibility of detecting the electron-positron pair as a signal of the QGP formation. Considering the results of this simulation, we designed and constructed the detector systems and performed the test experiments for the detection of electron-positron pair.