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

在宇宙诞生期间，人们推测宇宙处于一种未知的物质形式，称为夸克-胶子等离子体(QGP)。高能重离子碰撞提供了在实验室中重建QGP的独特机会。对碰撞中强子产生的系统研究对于基本理解这个复杂系统是至关重要的，这可能是QGP形成的特征。高分辨率飞行时间(TOF)计数器(分辨率为sigma；&lt；85ps)已经建立起来，它为强子的粒子识别提供了关键作用。用于高分辨率TOF测量的流水线读出电路是与哥伦比亚大学尼维斯实验室共同开发的。利用飞行时间系统，系统地测量了10AGeV、158AGeV和100+100AGeV能量的重离子碰撞中产生的强子。其中，对能量为158AGeV/c的PB+PB碰撞中粒子集体发射的研究是重要的。从对在目标快速区发射的质子的方位相关的逐个事件的分析中，第一次在这个重离子碰撞能量下观察到定向流。定向流的大小比10A GeV时观测到的小，也比RQMD模型所预测的小。这一事实可能表明状态方程的软化，这是夸克胶子等离子体形成的特征。对于由定向流测量确定的反应平面，在中快度附近观察到了K+、介子和质子的方位相关。在半中心碰撞中，K+介子优先发射出反应平面，而介子和质子则发射到反应平面。结果表明，除了集体流动效应外，K+的释放还受到介质势能效应的影响。

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.