= Quartic W-photon-Z gauge coupling
{title2=$g_{WWZ\gamma}=eg\cos\theta_W$}
The non-Abelian electroweak gauge kinetic term contains a $W^+W^-W^3W^3$ interaction. Substituting $W^3=\sin\theta_W A+\cos\theta_W Z$ gives a mixed four-gauge-boson vertex with coupling magnitude $g^2\sin\theta_W\cos\theta_W=eg\cos\theta_W$. It can emit both a <photon> and a <Z boson> at one vertex. A one-loop flavour-changing <quark> graph with two quark-W vertices and this quartic vertex consequently has three vertices despite having four external particles.
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