欧洲核子研究中心发现新物质形态——夸克胶子等离子体存在的证据

介绍和评述了欧洲核子研究中心（CERN）的相对论重离子碰撞的重要实验结果及其物理解释。碰撞初期形成了高温高密系统。能量密度达到了产生夸克胶子等离子体（QGP）的阈值,低质量双轻子增长、J／ψ反常压低和奇异粒子增长等实验现象可以用形成了QGP来解释。但目前的实验还不足以证明已经产生了QGP。     

高能重离子物理进展与展望

介绍了高能重离子碰撞物理的基本要领指出这一领域的中心课题是研究实现夸克一胶子等离子体相变的条件，文章了对各种可能的ＱＧＰ相变信号的理论分析及实验探测的最新进展，并展望了该领域实现了理论研究进展的未来前景。     

柱形夸克胶子等离子体的2π干涉学检测

在2п干涉学中,对不同的п源2п,关联函数可以是不同的. 如果在相对论重离子碰撞中出现柱形夸克胶子等离子体,п介子将从柱形夸克胶子等离子体表面发射,此时2п关联函数将出现一种特殊的振荡行为.这种振荡行为可以用来探明夸克胶子等离子体的存在性,同时也是相对论重离子碰撞中出现夸克胶子等离子体的一个信号.     

相对论重离子碰撞中的软探针和硬探针

简要回顾了高能核碰撞中夸克胶子等离子体的软探针和硬探针的一些最新进展,主要内容集中在相对论重离子对撞机和大型强子对撞机实验中各向异性集体流和喷注淬火的理论和唯象研究,对小系统中集体流的来源也做了简要的讨论。对于软探针,讨论了初态三维涨落和碰撞几何各向异性、相对论流体力学演化、末态各向异性集体流以及集体流的涨落、关联和纵向去关联等。通过与实验数据作系统的比较,可以探测重离子碰撞中夸克胶子等离子体的动力演化和各种输运性质。对于硬探针,集中讨论了部分子能量损失和喷注淬火对部分子味道的依赖性、重味夸克在夸克胶子等离子体中的强子化、整体喷注在核介质中的演化以及核介质对喷注的响应等。细致分析相关的观测量,可以帮助我们更全面地了解相对论核碰撞中喷注与核介质的相互作用以及重味粒子的生成。对于小系统,讨论初态和末态效应在解释小系统中轻强子和重味强子的集体流方面的贡献,这有助于我们理解大碰撞系统中集体流的起源成因。     

非静态球形夸克-胶子等离子体的2π干涉学的检测

提出了一种相对论重离子碰撞中强子物质生成时的π膨胀源模型. 得到了该模型中的小相对论动量区域的2π关联函数及表观π源参数R_a与实际π源参数R_e的关系. 这一关系与QGP生成时的关系有所区别. 这一区别可以用来检测相对论重离子碰撞中产生的非静态球形夸克-胶子等离子体的存在性. 两个关系也显示了一种标度行为.     

Thermal Charmed Quark Contribution to Dileptons in Chemically Equilibrating Quark—Gluon Matter

We find that in a chemically equilibrating baryon-rich quark-gluon matter, due to the slow cooling rate, high initial temperature, large gluon density as well as large fusion cross section of gg→cc^- in the intermediate mass region, the gluon fusion gg→cc^- provides a dominant contribution to dileptons with intermediate masses, resulting in the significant enhancement of intermediate mass dileptons.     

A Comparison of Different Measures for Dynamical Event Mean Transverse Momentum Fluctuation

Various measures for the dynamical event mean transverse momentum fluctuation are compared with the real dynamical fluctuation using a Monte Carlo model. The variance calculated from the G-moments can reproduce the dynamical variance well, while those obtained by subtraction procedures are approximate measures for not very low multiplicity. Фpt, proposed by Gadzicki M and Mrowczynski S [Z. Phys. C 54 (1992)127], can also serve as an approximate measure after being divided by the square root of mean multiplicity.     

Medium Density and Parton Energy Loss in Gold-Gold Collisions from the Recombination Model

The pion production at mid-rapidity in gold-gold collisions at √s = 200 GeV is investigated by the parton recombination model. The density of the hot medium produced in the collisions and the hard parton energy loss effect are obtained by fitting the experimental data. Both of them are found to have power-law dependence on the number of binary collisions. The implication of such dependence on the nuclear modification factor is discussed.     

相对论重离子碰撞中的重夸克势

在高温高密的夸克胶子等离子体(QGP)中,重夸克的相互作用势会受到周围部分子的影响而发生改变。一般认为,重夸克偶素的相互作用势的实部应该介于重夸克偶素的自由能F和内能U之间,且具体形式依赖于温度。而重夸克偶素的虚部相互作用势则来自朗道阻尼效应。基于含时薛定谔方程模型,采用不同的重夸克相互作用势,研究重夸克偶素的波函数在QGP中的演化,并计算重夸克偶素的核修正因子。通过同时考虑冷核效应和热核效应,把理论计算的底夸克偶素的核修正因子与实验数据对比,发现当重夸克偶素的实部相互作用势接近内能U时,能够较好地解释实验现象,并能看到不同底夸克偶素的连续压低现象,即更高激发态的底夸克偶素由于其自身束缚能更小而更容易被解离。薛定谔方程模型,有利于建立有限温度重夸克势与实验观测量之间的直接联系,并确定相互作用势的形式。     

J／Φ Dissociation in QGP via Multi-gluon Absorption

We propose that the suppression of J/Φ production in relativistic heavy ion collisions may be explained by that J/Φ dissociates via absorbing multi-gluons in the environment of quark-gluon-plasma (QGP) where abundance of gluons is expected.     

Spatial anisotropy: a method to study anisotropic flow in heavy ion collisions

We introduce a method to study anisotropic flow parameter v<,n> as a collective probe to Quark Gluon Plasma in relativistic heavy ion collisions. The emphasis is put on the use of the Fourier expansion of initial spatial azimuthal distributions of participant nucleons in the overlapped region. The coefficients ε<,n> of Fourier expansion are called the spatial anisotropy parameter for the n-th harmonic. We propose that collective dynamics can be studied by v<,n>/ε<,n>. In this paper, we will discuss in particular the second (n=2) and the fourth (n=4) harmonics.     

Multifragmentation and dynamics in heavy ion collisions

A midrapidity zone formed in heavy-ion collisions has been investigated through special selections of light particles and intermediate mass fragments detected in the reaction ³⁵Cl on ¹²C at 43 MeV/nucleon and the reactions ⁵⁸Ni on ¹²C, ²⁴Mg, and ¹⁹⁷Au at 34.5 MeV/nucleon, and of neutron energy spectra measured in the reaction ³⁵Cl on ^natTa. Properties of the observables have been examined to characterize the neck-like structure formed between the two reaction partners. For the HERACLES collaboration.     

Collective expansion and hadronization in high energy heavy ion collision experiments

An overview of research status of soft physics in high energy heavy-ion collision experiments and recent experimental results are presented.     

System-size scan of dihadron azimuthal correlations in ultra-relativistic heavy ion collisions

System-size dependence of dihadron azimuthal correlations in ultra-relativistic heavy ion collision is simulated by a multi-phase transport model. The structure of correlation functions and yields of associated particles show clear participant path-length dependences in collision systems with a partonic phase. The splitting parameter and root-mean-square width of away-side correlation functions increase with collision system size from ¹⁴N + ¹⁴N to ¹⁹⁷Au + ¹⁹⁷Au collisions. The double-peak structure of away-side correlation functions can only be formed in sufficient “large” collision systems under partonic phase. The contrast between the results with partonic phase and with hadron gas could suggest some hints to study onset of confinement.     

Azimuthal correlations between directed and elliptic flow in heavy ion collisions

A method for investigating the azimuthal correlations between directed and elliptic flow in heavy ion collisions is described.The transverse anisotropy of particle emission at AGS energies is investigated within the RQMD model.It is found that the azimuthal correlations between directed and elliptic flow are sensitive to the incident energy and impact parameter.The fluctuations in the initial stage and dynamical evolution of heavy ion collisions are not negligible.     

Prompt contributions to the dilepton yield in heavy ion collisions

The mass spectrum is calculated for those dileptons which are produced in the early phase of a heavy ion collisions via the direct production NN → l⁺l⁻X and via the Compton process GN → l⁺l⁻X with prompt gluons radiated in preceding NN interactions. Both mechanisms produce a mass spectrum which decreases steeply with invariant mass of the l⁺l⁻ pair and which is below the CERES data for Pb-Au collisions by about one order of magnitude. Received: 1 December 1999 / Revised version: 18 February 2000