Systematic measurement of hadron production for the study of Quark-Gluon Plasma

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During the period of the birth of the Universe, it is speculated that the universe was in an unknown from of matter, called quark-gluon plasma (QGP). Unique opportunity to re-create the QGP in the laboratory is provided by high energy heavy ion collisions. Systematic study of hadron production in the collisions is essentially important for the basic understanding of the complex system, which might characterize the formation of the QGP.High resolution time-of flight (TOF) counters (resolution in sigma ; <85 ps), which provides the key role for the particle identification of the hadron, has been constructed. Pipeline readout circuits for high resolution TOF measurement have been developed together with Nevis Lab., Columbia University. Utilizing the TOF system, systematic measurement of the produced hadrons has been carried out in heavy ion collisions at the energy of <10 A GeV, 158A GeV and 100+100 AGeV as planned.Among several achievements, study of collective emissions of particles in PB+PB collisions at the energy of 158 A GeV/c is important. From an event-by-event analysis of the azimuthal correlation of protons emitted at the target rapidity region, directed flow has been observed as a first time at this energy of heavy ion collisions. The magnitude of the directed flow is found to be smaller than those observed at 10A GeV and also smaller than those predicted by RQMD model. This fact may suggest a softening of equation of state, which characterizes the formation of Quark Gluon Plasma. With respect to the reaction plane, determined from the measurement of the directed flow, azimuthal correlation of K+, pion and proton near mid-rapidity have been observed. In semi-central collisions, K+ mesons are found to be preferentially emitted out of the reaction plane while pions and protons are emitted in the reaction plane. The result suggest that the K+ emission is influenced by in-medium potential effects in addition to the collective flow effects.