Radiative corrections to Γ(Z→bb¯) 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 briefly discuss the model and show that radiative corrections (involving these scalars) to the branching ratio $R_b=\Gamma (Z\rightarrow b\bar b)/\Gamma (Z\rightarrow {\rm hadrons})$ are negative and thus place a stringent lower bound on the masses of the colored scalars. This turns out to be $\sim 400$ GeV with $m_t=150$ GeV and increases quadratically with $m_t$. It is emphasized that $R_b$ is a parameter that is well-determined experimentally and that theoretical estimates are relatively free from uncertainties emanating from hadronic corrections and are comparatively insensitive to the value of the top mass.Comment: 9 pages + 3 figures (available upon request), TeX file. Saha Institute Preprint no. SINP-TNP/93-22 (December 1993