Charmonium from Statistical Hadronization of Heavy Quarks: A Probe for Deconfinement in the Quark-Gluon Plasma
Charmonium from Statistical Hadronization of Heavy Quarks: A Probe for Deconfinement in the Quark-Gluon Plasma
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DOI：
10.1007/978-3-642-01539-7_14
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发表时间：
2009-01
期刊：
影响因子：
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通讯作者：
P. Braun-Munzinger;P. Braun-Munzinger;J. Stachel
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文献类型：
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作者：
P. Braun-Munzinger;P. Braun-Munzinger;J. Stachel
对超相对论核-核碰撞中强子产生的研究揭示了热产生机制的令人信服的证据。特别是，对由从 AGS 到 RHIC 能量的轻（u、d、s）价夸克组成的强子产率的研究表明 [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12] 强子多重性可以在强子化学平衡方法的框架中定量描述。在该模型中，唯一的参数是热量：化学冻结温度 T、化学势 μ 以及火球体积 V（如果适用）；最近的评论请参阅[13]。两个高能重核碰撞形成的系统演化的基本图景如下：在碰撞的早期阶段，部分子在可通过微扰量子色动力学（QCD）描述的硬碰撞中释放出来。粒子系统随后达到平衡，即达到（近似）局部动量各向同性，同时始终以光速沿光束方向扩展。该系统称为火球，其特征在于温度等热参数和状态方程。最终横向扩张也会建立起来。膨胀的火球冷却，其温度下降为 T∝ τ− 1/3（或由于横向膨胀而稍微加快）。最终火球到达夸克物质和强子物质之间的相界，并且部分子自由度被转换为强子自由度。自由度相应减少超过 3 倍，因此强子化过程中体积必须相应增大。在等于或低于临界温度的某个温度下，强子产量被冻结。这就是所谓的化学冻结，相应的热参数 T 和 μ 是根据前面第 1 段中讨论的强子产量分析确定的。现在的强子火球可能会进一步膨胀和冷却，直到弹性碰撞发生改变动量分布。此点称为热冻结。在此之后，可能仍然存在一些残余相互作用（例如库仑相互作用）和弱衰变。由此产生的动量和粒子类型在探测器中进行测量。
Investigation of hadron production in ultra-relativistic nucleus-nucleus collisions has revealed convincing evidence for a thermal production mechanism. In particular, the study of yields of hadrons composed of light (u, d, s) valence quarks from AGS up to RHIC energies has shown [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12] that hadron multiplicities can be described quantitatively in the framework of a hadro-chemical equilibrium approach. Within this model the only parameters are thermal quantities: the chemical freeze-out temperature T, the chemical potentials μ and, if applicable, the fireball volume V; for a recent review see [13]. The underlying picture of the evolution of the system formed in a collision between two heavy nuclei at high energies is the following: In the early phase of the collision partons are liberated in hard collisions describable by perturbative quantum chromo dynamics (QCD). The partonic system subsequently equilibrates, ie reaches (approximate) local momentum isotropy, all the while expanding in beam direction with velocity of light. This system is called the fireball and is characterized by thermal parameters such as a temperature and by an equation of state. Eventually also transverse expansion builds up. The expanding fireball cools with its temperature dropping as T∝ τ− 1/3 (or slightly faster due to transverse expansion). Eventually the fireball reaches the phase boundary between quarkmatter and hadronic matter and the partonic degrees of freedom are converted into hadronic degrees of freedom. The corresponding reduction in degrees of freedom is more than a factor of 3 and therefore the volume has to grow accordingly during hadronization. At some temperature equal or below the critical temperature hadron yields are frozen in. This is what is called the chemical freeze-out and the corresponding thermal parameters T and μ are determined from the analysis of hadron yields as discussed in the previous paragraph 1. The now hadronic fireball may expand and cool further until elastic collisions seize to change the momentum distributions. This point is called thermal freeze-out. After this point there may still be some residual interactions (eg Coulomb interaction) and weak decays. The resulting momenta and particle types are measured in the detectors.