【作者】 王晓东；

【导师】 王恩科；

【作者基本信息】 华中师范大学 ， 理论物理， 2007， 硕士

【摘要】 量子色动力学（QCD）是用非阿贝尔规范理论研究基本粒子之间的强相互作用。它预言在高温高密极端条件下，普通强子物质会发生退禁闭相变到夸克胶子等离子体（QGP）相，原来被束缚在强子体系内部的夸克和胶子退禁闭成自由的QGP物质。研究高温高密的核物质以及寻找夸克胶子等离子体（QGP），实验上最有效的方法是通过极端相对论性重离子碰撞来实现的。高能重离子在相互发生碰撞的过程中，巨大的粒子动能被沉积在核-核碰撞的区域内，并转化为热能，形成极端高温高密环境，致使强子解除禁闭，形成QGP新物态。近年来，通过ACS，SPS以及RHIC实验，人们已经积累了大量丰富的实验数据。通过对实验数据分析和研究，有迹象表明强耦合的夸克胶子等离子体这一物质形态已经在RHIC实验中形成。在RHIC能区研究夸克胶子等离子体是否形成的信号中，由于碰撞能量很高(s^1/2=200GeV)，与碰撞硬过程相关的硬探针信号，特别是部分子喷注能量损失成为目前QGP信号研究的热点。部分子喷注损失能量必然减少部分子碎裂为强子的产额，导致与相同能量的核子-核子碰撞相比，核-核碰撞产生的大横动量强子谱被压低。在STAR和PHENIX实验中，通过比较核-核碰撞和核子-核子碰撞中领头粒子的产额，分别都观测到领头的中性π⁰产额压低现象。通过测量核-核碰撞中的核修正因子，我们可以更清楚看到能量损失在核-核碰撞中对强子谱压低的影响。通过观察这些实验结果，我们相信在核-核中心碰撞中形成了高温高密的强相互作用物质，而擦边碰撞的核-核碰撞中就没有形成。值得注意的是，RHIC的实验数据还表明强子谱的压低与碰撞产生的系统的大小有关。这一实验现象不仅证明存在部分子喷注损失能量的事实，而且还能说明能量损失与介质的尺寸大小有直接的关系。本文基于微扰量子色动力学（pQCD）的部分子模型，在领头阶（LO）近似下分别计算了质子-质子（p+p）碰撞，金-金（Au+Au）和铜-铜（Cu+Cu）不同对心度碰撞产生π⁰的强子谱和相应的核修正因子R_AA。在考虑部分子喷注能量损失（即喷注淬火）效应后的理论计算表明，与相同质心系能量s^1/2=200GeV的p+p碰撞相比，Au+Au或Cu+Cu不同对心度碰撞产生的强子谱被压低，被压低的程度还与依赖系统大小的能量损失有强烈的关系。系统越大，部分子喷注的能量损失越大，导致强子谱被压低得越多，R_AA越小。系统越小，部分子喷注的能量损失越小，擦边碰撞时就几乎没有能量损失，R_AA近似等于1。理论计算的数值结果和实验数据相符合，说明能量损失机制的可靠性和真实性。更多还原

【Abstract】 Quantum Chromo-Dynamics （QCD）, the theory of strong interaction, predicts that nuclear matter undergoes a phase transition to a deconfined state of quark and gluon at extreme densities and temperatures. The condition of extreme high energy density and temperature could be created in high energy heavy-ion collision experiment, then the study of the new matter state-Quark-Gluon Plasma （QGP） the goal that high energy and nuclear physics scientists all over the world have been searching for collaborative since several past decades.In high energy heavy ion collisions, the suppression of the yield of final state high transverse momentum hadrons is considered to be a important signal of exiting Quark-Gluon Plasma. In the process of nuclear-nuclear（AA） collisions, hard scattering is on the initial stage. High transverse momentum partons which come from hard scattering will propagate in the strong interaction medium and interact with matter. These partons are expected to lose their energy in medium by gluon bremsstrahlung, which is called jet quenching.In RHIC, through comparing the yield of leading particle in nuclear-nuclear collisions with that in nucleon-nucleon collision it shows that hadron spectrum of neutral pions are suppressed because of jet quenching or jet energy loss. Furthermore, the affect of the jet quenching or energy loss is more obvious through computing the nuclear modified factor. After studying the central and the peripheral collisions, we find that the hot and dense matter or QGP matter is formed in central AA collisions, but not in the peripheral AA collisions.Based on parton model in QCD, at the leading-order approximation we study theπ⁰ spectra and nuclear modification factor R_AA in Au + Au and Cu + Cu collisions with different centralities. Because of parton jet energy loss in heavy ion collision, large transverse momentum hadrons spectra are suppressed as compared to p + p collisions at the same energy. It is shown that the suppression has strong dependence on the system size which determines the amount of the jet energy loss. The bigger the system is, the larger parton energy loss is. It results in the much more suppression of the spectra and also the much less R_AA.更多还原