TOP-CHARM FLAVOR-CHANGING CONTRIBUTIONS TO THE EFFECTIVE BSZ VERTEX

We analyze the effects of a tree level flavor changing tcZ vertex indu ced by a mixing with new isosinglet Q = 2/3 quarks, on the effective b sZ vertex. We compute the contributions arising from the new electrowe ak 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, th e presence of a tcZ coupling cannot enhance the rates for the Z mediat ed flavor changing decays b --> s l(+)l(-) and b --> <s nu(nu)over bar >, implying that these processes cannot be used to set limits on the t cZ coupling. The additional effects of the heavy isosinglets are compa red with the well studied effects of new isodoublets appearing in mult i-generational models.