b
= b
{scope}
\Image[/past-exam-of-the-quantum-field-theory-course-of-the-university-of-oxford/2022-fermion-tree-channels.png]
{title=Original tree diagrams with external momentum labels and fermion-flow arrows. The upper row gives two-fermion t and u exchange; the lower row gives fermion–antifermion t exchange and s annihilation. Dashed lines denote the auxiliary scalar.}