Color-magnetic correlations in SU(2) and SU(3) lattice QCD

TL;DR

This study investigates color-magnetic correlations in SU(2) and SU(3) lattice QCD, revealing distinct behaviors of perpendicular and parallel correlations and their cancellations, with implications for understanding the QCD vacuum.

Contribution

It provides a detailed analysis of color-magnetic correlations and their decomposition into gluon field terms, highlighting differences between SU(2) and SU(3) and the cancellation effects in the QCD vacuum.

Findings

Perpendicular magnetic correlations are negative except at zero distance.

Parallel magnetic correlations are always positive.

Significant cancellation occurs in the sum of field-strength correlations at large distances.

Abstract

We study the two-point field-strength correlation $g^2 \langle G_{\mu\nu}^a(s)G^b_{\alpha\beta}(s') \rangle$ in the Landau gauge in SU(2) and SU(3) quenched lattice QCD, as well as the gluon propagator $g^2 \langle A_\mu^a (s)A_\nu^b(s') \rangle$. The Landau-gauge gluon propagator $g^2 \langle A_\mu^a (s)A_\mu^a(s') \rangle$ is well described by the Yukawa-type function $e^{-mr}/r$ with $r\equiv |s-s'|$ for $r=0.1-1.0~{\rm fm}$ in both SU(2) and SU(3) QCD. Next, motivated by color-magnetic instabilities in the QCD vacuum, we investigate the perpendicular-type color-magnetic correlation, $C_{\perp}(r) \equiv g^2\langle H_z^a(s)H_z^a(s + r \hat \perp)) \rangle$ ($\hat \perp$: unit vector on the $xy$-plane), and the parallel-type one, $C_{\parallel}(r) \equiv g^2 \langle H_z^a(s)H_z^a(s + r \hat \parallel) \rangle$ ($\hat \parallel$: unit vector on the $tz$-plane). These two quantities reproduce all the correlation of $g^2\langle G^a_{\mu\nu}(s)G^b_{\alpha\beta}(s')\rangle$, due to the Lorentz and global SU($N_c$) color symmetries in the Landau gauge. Curiously, the perpendicular-type color-magnetic correlation $C_{\perp}(r)$ is found to be always negative for arbitrary $r$, except for the same-point correlation. In contrast, the parallel-type color-magnetic correlation $C_{\parallel}(r)$ is always positive. In the infrared region of $r \gtrsim 0.4~{\rm fm}$, $C_{\perp}(r)$ and $C_{\parallel}(r)$ strongly cancel each other, which leads to a significant cancellation in the sum of the field-strength correlations as $\sum_{\mu, \nu} g^2\langle G^a_{\mu\nu}(s)G^a_{\mu\nu}(s')\rangle \propto C_{\perp}(|s-s'|)+ C_{\parallel}(|s-s'|) \simeq 0$. Finally, we decompose the field-strength correlation into quadratic, cubic and quartic terms of the gluon field $A_\mu$ in the Landau gauge.