【作者】 刘志旭；

【导师】 刘峰； 许怒；

【作者基本信息】 华中师范大学 ， 理论物理， 2005， 博士

【摘要】 人类的好奇心驱使着我们几千来探索同一个问题:物质的最基本的组成部分是什么?由于缺少实验的支持,对这个问题最初的回答只能是哲学上的观点。只是在最近几百年来,随着近代科技的发展,我们对这个问题的认识才越来越深入!1883年英国科学家约翰·道尔顿发展了古希腊哲学家德谟克利特的朴素的原子观点。原子又是由电子和原子核组成。原子核包括质子和中子。但随后又发现的一大批新的粒子使人们认识到这些粒子可能由更基本的结构组成,这就是夸克和胶子。并且发展了量子色动力学（QCD）来描述它们之间的相互作用:强相互作用。目前我们所了解的物质基本组成部分是夸克/胶子和轻子。QCD的两个基本特征是渐进自由和夸克禁闭,即夸克之间的相互作用在很小距离上变弱以及夸克不能以自由粒子存在,而只能禁闭在强子物质内。QCD理论预测在极高温度和（或）重子数密度下,强子物质可能会解除禁闭而以夸克胶子等离子体（QGP）的形式存在。在这种新的物质形态内,夸克和胶子可以在较大的（超出核子）范围内运动。 夸克胶子等离子体可能存在于宇宙大爆炸早期阶段（很高的温度）以及中子星（重子数密度很高）内。然而这两种情况要么是发生在过去,要么就是离我们距离太远,因此没法直接探测。在实验室里我们让两束高能重离子（高重子数密度）束流以极高的能量（高温）对撞,从而产生能量密度很高以及生存时间比较长的强相互作用物质,QGP有可能在这种环境下存在。从最早的伯克力（Berkeley）的Bevalac,到欧洲核子研究中心（CERN）的超级质子同步加速器（SPS）,以及布鲁克海汶国家实验室（BNL）的交变梯度同步加速器（AGS）,人们已经进行了很多有意义的工作,对高温高密度下的核物质的性质进行了大量的研究。2000年夏天,相对论重离子对撞机（RHIC）在BNL开始投入运行。RHIC的最高质心系能量（核核对撞）(S_NN)^1/2=200 GeV为目前最高的能量。预计2007年在CERN运行的大型强子对撞机（LHC）将有更高的质心系能量(PP对撞(S_NN)^1/2=14 TeV,重离子对撞(S_NN)^1/2=5.4 TeV)。 由于洛伦兹收缩,两个高速飞行的重离子就像两个“铁饼”一样对撞。在较低能量的情况下,两个“铁饼”会简单的弹开。随着碰撞能量的不断升高,它们会开始穿透对方从而形成高度激发的核物质并伴随这碎裂的核碎片以及其他新的粒子的更多还原

【Abstract】 The mid-rapidity proton and anti-proton yields are presented for the SNn = 200 GeV Au+Au data sets which were taken by the Solenoidal Tracker at RHIC (STAR) in 2001 run. The results are from transverse momentum range 0.4 < pt < 1.05 GeV/c and rapidity range |y| < 0.5 by using the energy loss in the Time Projection Chamber (TPC). The measured transverse momentum distribution becomes more convex from peripheral to central collisions for both proton and anti-proton implying the strong collective expansion at the early stage of the collision. The measured rapidity distributions of both proton and anti-proton are fiat within |y| < 0.5 indicating a boost invariant region around mid-rapidity. p/p≈0.8 is independent of the measured rapidity region |y| < 0.5 and decreases slightly from peripheral collision (≈ 0.85) to central collisions (≈ 0.80). It’s still not net baryon free at RHIC energy. The slight decrease in the p/p ratio reflects the rich collision dynamics at RHIC: both initial baryon transfer and final stage hadronic rescatterings are important for the observation.The kinetic freeze-out conditions are extracted by applying a thermal + radial flow fit to the proton data and <pf> are calculated by extrapolating the measured spectra with the model. The kinetic freeze-out temperature decreases from peripheral (≈135 MeV) to most central collision (≈89 MeV). They are all smaller than the chemical freeze-out temperature (≈160±5 MeV) indicating an additional hadronic rescattering phase after the chemical freeze-out in Au + Au collisions at RHIC. The transverse flow velocity increases from peripheral to central collision. The central-ity dependence of (pt) for different particles (π, K, P) confirm this conclusion. The difference between the <pt> of different particles (π, K, P) increases as centrality increases, this indicates that the development of collective flow is stronger in central collisions than in peripheral collisions.The same results are also calculated by employing a transport model: Rela-tivistic Quantum Molecular Dynamics (RQMD). The spectra of different particles (π, K, P) are not following the so called m_t scaling. While the <pt> of them show similar trend with the experiment data despite of the underestimation of the absolute value. Furthermore, the underestimation of the absolute value might indicate early flow development. The earlier freeze-out of multi-strange particles (φ, Ξ, Ω) is demonstrated with this model from the freeze-out time and radius distribution of these particles. The measurements of these particles are necessary to confirm this. By applying thermal model fit to the spectra from this model, the same trend of kinetic freeze-out condition as in data is observed. The effects of resonance decay on the thermal fit parameters are also studied by letting the resonance particles decay with Pythia. The effect is small under this model’s framework. Afterswitch off the rescattering in this model, the violation of mt scaling and central-Ity dependence of (pt) disappear. This indicates the importance of rescattering in heavy-ion collisions. However, only hadronlc interactions are included in RQMD. Such hadronlc interaction does not generate enough collective flow comparing to data. This demonstrates that partonlc collectivity is needed In the heavy ion collisions at RHIC. The realization of the partonlc collectivity is important toward the understanding of the partonlc equation of state in high-energy nuclear collisions.更多还原