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 = 23 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 b → sℓ+ℓ− and b → sνν-, 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.