【作者】 杨驰；

【导师】 李澄； 阮丽娟；

【作者基本信息】 中国科学技术大学 ， 粒子物理与原子核物理， 2014， 博士

【摘要】 大爆炸理论预言在大爆炸发生后约几十微秒的宇宙早期可能会产生一种特殊的物质形态-夸克胶子等离子体(QGP)。格点QCD计算预言了在高温和低重子密度的条件下的从普通强子物质到这种夸克解禁闭的局部热平衡物质的相变。位于美国布鲁克海文国家实验室(BNL)的相对论重离子对撞机(RHIC)通过高能重离子碰撞提供了寻找QGP物质形态,研究其行为和性质的实验环境。这种物质会以直接光子和双轻子的方式发射热辐射。作为电弱相互作用探针,因为不参与强相互作用,直接光子和轻子一旦产生在穿越相对论重离子碰撞产生的高温高密物质时只受到很小的相互作用。它们被认为能够携带从各个碰撞演化过程中的信,对其的研究能够反映碰撞系统最直接最纯净的信息。本文给出了位于RHIC的螺旋管径迹探测器(STAR)中直接虚光子的首次研究结果。这也是在高横动量下首次基于虚光子方法的直接光子测量。测量是基于2010年与2011年采集的每核子对质心能量200GeV的金核金核碰撞数据,给出了直接虚光子的不变产额及其与单举光子的比例。这种基于虚光子方法测量的直接光子被称为直接虚光子。在横动量区间1<pT<10GeV/c的测量来源于对低不变质量区间的双电子的测量。2010年安装完毕的飞行时间谱仪(TOF)提供了从低到中横动量区间的干净的电子鉴别,使得双电子的测量成为可能。数据分析结果显示直接虚光子的产额谱在低横动量区间比从质子质子碰撞结果得到的预期有明显的升高。这种升高说明了在此区间中存在物质热化的贡献。不变产额谱与理论计算得到的预期相符合,其包含QGP,强子气体(hadron gas)和原初产生的贡献。在高横动量区间,直接虚光子的产额谱符合从质子质子碰撞结果得到的预期。这说明在此区间来自于物质热辐射的贡献非常微弱。对直接虚光子的测量提供了两个动力学区间：1)可以对热化物质进行研究的低横动量区间；2)可以研究原初过程中部分子的硬散射的高横动量区间。一个与初始温度相关的逆坡度参数(inverse slope parameter)也可以从不变产额谱中得到。更多还原

【Abstract】 The Big-Bang theory indicates that the Quark Gluon Plasma (QGP) is formed at the very universe in a few tens of microseconds. Lattic QCD predicts a phase transition from hadronic matter to this deconfined and locally thermalized matter at high temperature and low baryon density. Relativistic Heavy Ion Col-lider (RHIC) at Brookhaven National Laboratory (BNL) provides an opportunity to study this strongly coupled QGP. The research of the behavior and proper-ty of QGP is a very interesting topic for physicist. This medium is expected to emit thermal radiation which is in the form of direct photons and dileptons. As electroweak probes, which do not suffer strong interaction, direct photon and lep-ton will traverse the hot and dense medium created by heavy ion collisions with minimal interactions once they are produced. They are believed to bring the in-formation from all the evolution steps. The research on these probes will provide the most direct and pure information.In this thesis, the first measurement on direct virtual photon from Solenoidal Tracker at RHIC (STAR) is reported. This measurement is also the first high transverse momentum(pT) result on direct photon via virtual photon method. This analysis is based on the data of sNN=200GeV Au+Au collisions taken from year2010run and year2011run (Run10and Runll). The invariant yield of direct virtual photon and the fraction of direct virtual photon versus inclusive photon are presented. This direct photon measured by virtual photon method is called direct virtual photon. Its production in the range of1<pT<10GeV/c is derived from the di-electron continuum in the low di-electron invariant mass region. The fully installed Time-of-Flight Detector (TOF) in2010provides clean electron identification from low to intermediate pT which enables the di-electron measurement. From the direct virtual photon invariant yield spectra, a significant en-hancement compared to the prediction based on p+p results is observed for1<pr<4GeV/c. The invariant yield is consistent with model prediction which includes the contributions from QGP, hadron gas and primordial produc-tion. In the high pT range, the invariant yield is consistent with the Ncoll scaled p+p results. This indicates a negligible contribution from thermal radiation. The study on the direct virtual photon provides two kinematic ranges:the low PT range where the thermal matter can be studied, the high pT range where the hard parton scattering from primordial step can be studied. The inverse slope pa-rameter which is related to the initial temperature is obtained from the invariant yield spectra.更多还原