Flavor-changingZdecays: A window to ultraheavy quarks?

We study flavor-changing $Z$ decays into quarks, $Z\ensuremath{\rightarrow}Q+\overline{q}$, in the standard SU(2)\ifmmode\times\else\texttimes\fi{}U(1) theory with sequential generations. Such decays occur in higher-order electroweak interactions, with a probability growing as the fourth power of the mass of the heaviest (virtual) quark mediating the transition. With the possible exception of $Z\ensuremath{\rightarrow}b\overline{s}$, these decay modes are generally very rare in the three-generation scheme. However, with four generations $Z\ensuremath{\rightarrow}{b}^{\ensuremath{'}}\overline{b}$ is observable if the ${t}^{\ensuremath{'}}$ mass is a few hundred GeV. Such decay modes could thus provide a glimpse of the ultraheavy-quark spectrum.