Gluon Condensate via Dirac Spectral Density: IR Phase, Scale Anomaly and IR Decoupling

Abstract

Quark and gluon scalar densities, $\langle \bar{\psi} \psi \rangle$ and $\langle F^2 \rangle$, reflect the degree of scale-invariance violations in SU(N) gauge theories with fundamental quarks. It is known that $\langle \bar{\psi} \psi \rangle$ can be usefully scale-decomposed via spectral density $\rho(\lambda)$ of Dirac modes. Here I give such formula for $\langle F^2 \rangle$, which reveals that gluon condensate is a strictly UV quantity. For the recently-found IR phase [1,2], where the infrared (IR) degrees of freedom separate out and become independent of the system's bulk, it implies that $\langle F^2 \rangle$ due to this IR part vanishes. Its glue thus doesn't contribute to scale anomaly of the entire system and is, in this sense, scale invariant consistently with the original claim. Associated formulas are used to define IR decoupling of glue, which may serve as an alternative indicator of IR phase transition. Using the simplest form of coherent lattice QCD, we express the effective action of full QCD entirely via Dirac spectral density.