a (source code)
= a
{scope}
The three allowed <tree-level Feynman diagrams> are shown below, using physical-particle <Feynman propagators> in <unitary gauge>. Solid arrows follow fermion number and the wavy lines denote the charged vector <Feynman propagator>.
\Image[/past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2017/iii/paper-305-charged-current-diagrams.png]
{title=Charged-current interaction diagrams}
{description=The allowed s-channel, t-channel and semileptonic <weak charged current> graphs.}
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