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

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.