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arXiv 2403.00747hep-thhep-lathep-ph
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正则量子化中的QCD theta参数

The QCD theta-parameter in canonical quantization

Wen-Yuan Ai, Bjorn Garbrecht, Carlos Tamarit

AI总结:

本文在四维欧几里得环面的纯Yang-Mills理论中通过正则量子化与规范固定研究QCD theta参数,证明受约束Hilbert空间保持CP对称并可推广至Minkowski时空。

摘要:

本文在四维欧几里得环面给出的时空中,研究了纯Yang-Mills理论中QCD theta参数的作用。虽然在这一设定中避免了引入可能非物理的边界条件,但必须说明应如何执行对拓扑扇区的求和。为了与实时观测量建立联系,我们将配分函数理解为对正则密度矩阵的求迹。于是该系统对应于空间三维环面上有限温度的系统。执行求迹运算需要正则量子化和规范固定。固定规范并要求保持Hamilton量的Hermite性,会导致物理波泛函的Hilbert空间受到限制,这一限制推广了由强加Gauss定律导出的约束。因此,我们发现,在一种内积下,Hilbert空间中的态可以被适当地归一化;该内积对由规范势表示的每个物理构型积分一次,且仅积分一次。从受约束Hilbert空间导出的观测量不会破坏电荷-宇称对称性。我们注意到,在Hamilton量中对任意theta值都存在的一个理论的精确隐藏对称性,在这个受约束空间中实际上被提升为宇称守恒。这些结果是在环面上推导的,以避免引入边界条件;当考虑所有可能的规范变换时,它们也可推广到Minkowski时空。

英文摘要:

The role of the QCD theta-parameter is investigated in pure Yang-Mills theory in the spacetime given by the four-dimensional Euclidean torus. While in this setting the introduction of possibly unphysical boundary conditions is avoided, it must be specified how the sum over the topological sectors is to be carried out. To connect with observables in real time, we perceive the partition function as the trace over the canonical density matrix. The system then corresponds to one of a finite temperature on a spatial three-torus. Carrying out the trace operation requires canonical quantization and gauge fixing. Fixing the gauge and demanding that the Hermiticity of the Hamiltonian is maintained leads to a restriction of the Hilbert space of physical wave functionals that generalizes the constraints derived from imposing Gauss' law. Consequently, we find that the states in the Hilbert space are properly normalizable under an inner product that integrates over each physical configuration represented by the gauge potential one time and one time only. The observables derived from the constrained Hilbert space do not violate charge-parity symmetry. We note that an exact hidden symmetry of the theory that is present for arbitrary values of theta in the Hamiltonian is effectively promoted to parity conservation in this constrained space. These results, derived on a torus in order to avoid the introduction of boundary conditions, also carry over to Minkowski spacetime when taking account of all possible gauge transformations.

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