GIM suppression of top-quark neutral-current decays (${\mathcal{A}}_{t\to qV}\propto {\sum }_i{V}_{ti}{V}_{qi}^{\ast }F(m_i^2/m_W^2)$)

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.