探测QCD手性相变中的标度异常：新的临界点确定
Detecting scale anomaly in chiral phase transition of QCD: new critical endpoint pinned down
摘要：
标度反常是量子场论中的一个基本概念，对理解QCD的真空结构具有重要意义，并可能对热QCD中的手征相变以及手征和U（1）轴对称性有潜在贡献。虽然这应该是必不可少的，但从未在手征相变中观察到尺度异常的直接证据。我们提出了一种探测手征相变中尺度反常的方法，即电磁诱导的尺度反常：在具有超重奇异夸克的双味无质量QCD上施加弱磁场背景，首先观察手征交叉;第二步，调节奇夸克质量越来越小，观察到二阶手征相变，然后是无质量三味极限的一阶。因此，作为量子尺度反常证据的二阶手征相变是一个新的临界点。事实证明，这种电磁尺度异常在弱磁场区而不是强磁场区最有效。我们还简要地讨论了格点量子色动力学的可及性、在稠密物质系统中的应用前景以及对天体物理观测的启示，如热磁类QCD理论提供的引力波产生。
英文摘要：
Violation of scale symmetry, scale anomaly, being a radical concept in quantum field theory, is of importance to comprehend the vacuum structure of QCD, and should potentially contribute to the chiral phase transition in thermal QCD, as well as the chiral and U(1) axial symmetry. Though it should be essential, direct evidence of scale anomalies has never been observed in the chiral phase transition. We propose a methodology to detect a scale anomaly in the chiral phase transition, which is an electromagnetically induced scale anomaly: apply a weak magnetic field background onto two-flavor massless QCD with an extremely heavy strange quark, first observe the chiral crossover; second, adjusting the strange quark mass to be smaller and smaller, observe the second-order chiral phase transition, and then the first-order one in the massless-three flavor limit. Thus, the second-order chiral phase transition, observed as the evidence of the quantum scale anomaly, is a new critical endpoint. It turns out that this electromagnetic scale anomaly gets most operative in the weak magnetic field regime, rather than a strong field region. We also briefly address accessibility of lattice QCD, a prospected application to dense matter system, and implications to astrophysical observations, such as gravitational wave productions provided from thermomagnetic QCD-like theories.
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