Unusual Features of QCD Low-Energy Modes in IR Phase

Abstract: It was recently proposed that there is a phase in thermal QCD (IR phase) at temperatures well above the chiral crossover, featuring elements of scale invariance in the infrared (IR). Here we study the effective spatial dimensions, $d_{IR}$, of Dirac low-energy modes in this phase, in the context of pure-glue QCD. Our $d_{IR}$ is based on the scaling of mode support toward thermodynamic limit, and hence is an IR probe. Ordinary extended modes, such as those at high energy, have $d_{IR}=3$. We find $d_{IR}<3$ in the spectral range whose lower edge coincides with $\lambda_{IR}=0$, the singularity of spectral density defining the IR phase, and the upper edge with $\lambda_A$, the previously identified Anderson-like non-analyticity. Details near $\lambda_{IR}$ are unexpected in that only exact zero modes are $d_{IR}=3$, while a thin spectral layer near zero is $d_{IR}=2$, followed by an extended layer of $d_{IR}=1$ modes. With only integer values appearing, $d_{IR}$ may have topological origin. We find similar structure at $\lambda_A$, and associate its adjacent thin layer ($d_{IR} >\approx 2$) with Anderson-like criticality. Our analysis reveals the manner in which non-analyticities at $\lambda_{IR}$ and $\lambda_A$, originally identified in other quantities, appear in $d_{IR}(\lambda)$. This dimension structure may be important for understanding the near-perfect fluidity of the quark-gluon medium seen in accelerator experiments. The role of $\lambda_A$ in previously conjectured decoupling of IR component is explained.