QCD高温相\boldmath{$U_A(1)$}对称性破坏的微观起源
Microscopic Origin of \boldmath{$U_A(1)$} Symmetry Violation in the High Temperature Phase of QCD
摘要：
我们研究了狄拉克矩阵的低阶本征模，目的是更深入地了解反常$U_A(1)$对称性的温度依赖性。我们使用重叠算符来探测由高度改进的交错夸克的（2+1）味产生的动力学QCD位形。我们发现没有证据表明，即使在$1.5\,T_c$，$T_c$是手性交叉温度的本征值谱的红外区域的间隙开放。相反，我们观察到的近零本征模的积累。我们认为，这些近零的本征模是主要负责的轴对称仍然有效的异常破缺。在$1.5\,T_c$，这些近零本征模保持本地化，它们的分布是一致的稀释瞬子气体图片。在这个温度下，瞬子的平均尺寸是$0.223(8)\,\text{fm}$，它们的密度是$0.147(7)\,\text{fm}^{-4}$。
英文摘要：
We investigate the low-lying eigenmodes of the Dirac matrix with the aim to gain more insight into the temperature dependence of the anomalous $U_A(1)$ symmetry. We use the overlap operator to probe dynamical QCD configurations generated with (2+1)-flavors of highly improved staggered quarks. We find no evidence of a gap opening up in the infrared region of the eigenvalue spectrum even at $1.5\,T_c$, $T_c$ being the chiral crossover temperature. Instead, we observe an accumulation of near-zero eigenmodes. We argue that these near-zero eigenmodes are primarily responsible for the anomalous breaking of the axial symmetry still being effective. At $1.5\,T_c$, these near-zero eigenmodes remain localized and their distribution is consistent with the dilute instanton gas picture. At this temperature, the average size of the instantons is $0.223(8)\,\text{fm}$ and their density is $0.147(7)\,\text{fm}^{-4}$.
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