= Solution
{c}
There are \b[two] <one-loop Feynman diagrams>: the two orders of the labelled <photons> around a <top quark> <fermion loop>, with one <Yukawa interaction> insertion for the outgoing <Higgs boson>. They are added, rather than identified by a <Feynman-diagram symmetry factor>. Each has the same <color charge> trace $N_c=3$.
\Image[/past-exam-of-the-advanced-quantum-field-theory-course-of-the-university-of-oxford/2022-top-triangle-routings.png]
{title=The two photon orders and consistent directed loop momenta}
{description=Original <Feynman diagrams>. Solid arrows show <fermion flow> and the labelled internal four-<momenta>. Wavy external lines are incoming <photons>; the dashed external line is the outgoing <Higgs boson>. The two panels use independent dummy <loop momenta>.}
In the first routing, traverse the <fermion loop> from the $p_2$ insertion to the $p_1$ insertion with <momentum> $l$, then to the <Higgs boson> insertion with <momentum> $l+p_1$, then back to the $p_2$ insertion with <momentum> $l-p_2$. Conservation at the <Higgs boson> insertion gives $p_h=p_1+p_2$. The second routing exchanges $(p_1,\mu)$ and $(p_2,\nu)$: its three internal <momenta> are $l$, $l+p_2$ and $l-p_1$. Reversing the orientation is another way to represent this second ordering. Two vector insertions give no <Furry theorem> cancellation.
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