Radiative corrections to Γ(Z→b bar b) from colored scalars in a model with dynamical symmetry breaking

Isodoublet color-octet scalar bosons appear in the low-energy limit of a natural extension of the standard model in which the electroweak symmetry is broken by a t bar t condensate. We show that radiative corrections (involving these scalars) to the branching ratio Rb=Γ(Z→b bar b)/Γ(Z→hadrons) are negative and thus place a stringent lower bound on the masses of the colored scalars. This turns out to be ∼400 GeV with mt=150 GeV (and ∼700 GeV for mt=175 GeV) and increases quadratically with mt. It is emphasized that the parameter Rb is well-determined experimentally and that theoretical estimates are relatively free from uncertainties emanating from hadronic corrections; thus, it is able to single out the vertex correction sensitive to the nondecoupling effects induced by the heavy top quark