= GIM suppression of top-quark neutral-current decays
{c}
{title2=$\mathcal A_{t\to qV}\propto\sum_iV_{ti}V_{qi}^*F(m_i^2/m_W^2)$}
For $q=u,c$ and a neutral <photon> or <Z boson>, the <Loop diagram>[loop] runs over <Down quarks>[down-type quarks]. <CKM matrix>[CKM unitarity] cancels a common mass-independent <Loop diagram>[loop] contribution. The large <Bottom quark>[bottom] <mass> relative to down and strange <masses> normally makes the surviving <Bottom quark>[bottom] contribution largest, and $|V_{cb}|>|V_{ub}|$ favours final <Charm quark>[charm]. Adding a resolved <photon> to a $qZ$ final state introduces another electromagnetic coupling and three-body <Lorentz-invariant phase space>[phase space] without removing the <Flavor eigenstate>[flavour] cancellation. Some diagrams use the <Quartic W-photon-Z gauge coupling>[quartic gauge vertex], so the number of vertices need not increase.
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