= Gluon self-energy
{title2=$\Pi^{\mu\nu,ab}(k)$}
The proper two-point correction to the <gluon propagator> in <QCD>. At one loop it receives <quark>, <gluon>, <Faddeev-Popov ghost field> and four-gluon <tadpole diagram> contributions. The <Slavnov-Taylor identity> makes the total tensor transverse, $k_\mu\Pi^{\mu\nu,ab}=0$, although the individual <gluon> and <ghost loop> contributions need not be transverse. The sign of the symbol must be tied to whether the amputated insertion is defined as $i\Pi$ or $-i\Pi$.
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