= Quark-gluon vertex
{title2=$-ig_s\gamma^\mu T^a$}
Expanding $\bar q i\gamma^\mu D_\mu q$ gives a vertex joining a <quark>, its conjugate line and a <gluon>. In the convention $\mathcal L_{\rm int}=-g_s\bar q\gamma^\mu T^aqA_\mu^a$, its rule is the displayed matrix. The <gluon> changes the <color charge> state through the generator $T^a$ but preserves the <quark flavour>. With distinct flavours in $q\bar q\to q'\bar q'$, this permits the tree-level annihilation graph but not a flavour-changing exchange along one <quark> line.
Back to article page
