Top-Charm flavor changing contributions to the effective $bsZ$ vertex

We analyze the effects of a tree level flavor changing $tcZ$ vertex induced by a mixing with new isosinglet $Q=2/3$ quarks, on the effective $bsZ$ vertex. We compute the contributions arising from the new electroweak penguin diagrams involving one insertion of the $tcZ$ vertex. We show that a generalized GIM mechanism ensures the cancellation of the mass independent terms as well as of the new divergences. Unexpectedly, the presence of a $tcZ$ coupling cannot enhance the rates for the $Z$ mediated flavor changing decays \bsll\ and \bsnn, implying that these processes cannot be used to set limits on the $tcZ$ coupling. The additional effects of the heavy isosinglets are compared with the well studied effects of new isodoublets appearing in multi-generational models.