= Solution
A <Feynman diagram> specifies a term in the perturbative <scattering amplitude>, not a measured microscopic trajectory. Assign external <spinors> or <polarization vectors>, an interaction factor to each <Feynman vertex> and a <propagator> to each internal line; impose <four-momentum> conservation and integrate over independent <loop momenta>. Sum amplitudes for indistinguishable alternatives, then square, sum final <spins> and average unpolarized initial <spins>. A <decay rate> uses $d\Gamma=|\mathcal M|^2d\Phi_n/(2M)$ in <natural units>; a <cross section> also divides by incident flux. <interference> means probabilities of individual diagrams must not simply be added.
The electromagnetic <fermion> vertex is $f\bar f\gamma$, with coupling proportional to $eQ_f\gamma^\mu$; it preserves <fermion> flavour. There is no elementary three-photon or four-photon vertex in <quantum electrodynamics>. The <weak charged current> has $W\ell\nu$ and $Wu_id_j$ vertices, with a left-chiral projector and a <Cabibbo-Kobayashi-Maskawa matrix> element at the <quark> vertex. The <weak neutral current> has $Zf\bar f$ vertices with vector and axial couplings; it includes <neutrinos> and is flavour diagonal at tree level. Electroweak gauge interactions also include $WW\gamma$, $WWZ$ and quartic $WWWW$, $WWZZ$, $WWZ\gamma$ and $WW\gamma\gamma$ vertices. Higgs-fermion and Higgs-vector interactions complete the mass-related scalar sector.<Electroweak Model and Constraints on New Physics by J. Erler and A. Freitas (2024)>[§10.1, Eqs. 10.2–10.7 and discussion of bosonic interactions]{ref}
<Quantum chromodynamics> has $q\bar qg$ vertices proportional to $g_s\gamma^\mu T^a$, plus three-gluon and four-gluon <Feynman vertices> because <gluons> carry <color charge>. A quark-gluon vertex preserves <quark flavour>. Internal particles need not be on their <mass shells>, but <electric charge> and <four-momentum> are conserved at each vertex. The figures below show representative leading flavour topologies; binding and <hadronization> are implicit, and additional strong-interaction contributions can modify hadronic amplitudes.
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