On color confinement
On color confinement
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关于色彩的限制
DOI：
10.29172/87ca58a8a3164438a43036d3d0a0f6b6
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发表时间：
2006
期刊：
影响因子：
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通讯作者：
A. Giacomo
中科院分区：
文献类型：
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作者：
A. Giacomo
夸克可以通过使用弱电探针在短距离内探测到。它们从未作为自由粒子被观察到。自由夸克存在的实验上限可以在粒子数据组的报告中找到[1]。自然界夸克丰度 nq 与核子丰度 np 之比的上限为 nq np ≤ 10，与标准宇宙学模型 10 中的预期值进行比较。产生夸克或反夸克的包容截面 σp 的上限为 10cm，与无约束情况下的预期值 ≈ 10cm 进行比较。由于某种对称性，抑制因子 10 只能作为 nq = 0 且 σq = 0 的自然解释。这种现象称为颜色限制：QCD 中的渐近粒子只是无色的。那么显而易见的问题是：QCD 是否意味着颜色限制？如果是，那么通过什么机制实现？产生限制的对称性是什么？参考文献[2]首次推测哈格多恩极限温度[3]实际上可能对应于从强子到夸克和胶子气体（所谓的夸克胶子等离子体）的去限制相变。欧洲核子研究中心和布鲁克海文已经建立了重离子碰撞实验来检测这种转变。目前尚不清楚这一转变的确凿信号是什么，而且到目前为止还无法做出明确的声明。虚拟实验，即晶格理论的数值模拟，反而证明了去限制转变的存在。格子蒙特卡罗技术产生定义 QCD 的函数费曼积分的离散近似。如果晶格间距 a 与强子尺度 λ 相比较小，并且 λ 又比晶格尺寸 La 小得多，则数值估计可以很好地逼近函数积分。有限温度下的 QCD 也可以通过类似的技术进行模拟。配分函数 Tr[e H T ] 等于在时间方向上从 0 压缩到 1 T 的费曼积分，对于玻色子场具有时间周期性边界条件，对于费米子具有反周期条件。在公式中
Quarks can be detected by use of electroweak probes at short distances. They have never been observed as free particles. The experimental upper limits to the existence of free quarks can be found in Particle Data Group reports[1]. The upper limit to the ratio of the quark abundance in nature nq to that of the nucleons np is nq np ≤ 10 to be compared with the expected value in the Standard Cosmological Model 10. The inclusive cross section σp to produce a quark or an antiquark has upper limit 10cm to be compared with the value expected in the absence of confinement ≈ 10cm. A suppression factor of 10 can only have as a natural explanation that nq = 0 and σq = 0 due to some symmetry. This phenomenon is called color confinement : asymptotic particle in QCD are only colorless. The obvious question is then: does QCD imply color confinement, and, if so, by what mechanism? What is the symmetry which produces confinement? For the first time it was conjectured in ref[2] that the Hagedorn limiting temperature [3] could in fact correspond to a deconfining phase transition from hadrons to a gas of quarks and gluons (the so called Quark Gluon Plasma). Experiments colliding heavy ions have been set up at CERN and at Brookhaven to detect such transition. It is not clear what would be the smoking-gun signal for that transition and no clear statement can be done up to now. Virtual experiments, i.e. numerical simulations of the theory on the lattice have instead demonstrated the existence of a deconfining transition. Lattice Montecarlo techniques produce a discretized approximant to the functional Feynman integral which defines QCD . If the lattice spacing a is small compared to hadronic scale λ and λ is in turn much smaller than the lattice size La the numerical estimate can be a good approximation to the functional integral. Also QCD at finite temperature can be simulated by similar techniques. The partition function Tr[e H T ] is equal to the Feynman integral compactified from 0 to 1 T in the time direction, with periodic boundary conditions in time for bosonic fields, antiperiodic for fermions. In formulae