 GIM suppression of top-quark neutral-current decays

For $q=u,c$ and a neutral photon or Z boson, the loop runs over down-type quarks. CKM unitarity cancels a common mass-independent loop contribution. The large bottom mass relative to down and strange masses normally makes the surviving bottom contribution largest, and $|V_{cb}|>|V_{ub}|$ favours final charm. Adding a resolved photon to a $qZ$ final state introduces another electromagnetic coupling and three-body phase space without removing the flavour cancellation. Some diagrams use the quartic gauge vertex, so the number of vertices need not increase.