THE ZB(B)OVER-BAR VERTEX - IMPLICATIONS FOR THE STANDARD-MODEL AND BEYOND

Electroweak precision data are analysed in extensions of the Standard Model with modifications of the vector boson propagators and the Zbb couplings. Assuming general vector and axial vector couplings between b quarks and the Z boson, we derive the specific Zbb vertex contributions in terms of the partial widths Gamma(<b(b)over bar>) - Gamma(<d(d)over bar>) from the data. Within special classes of models (models with modifications exclusively of the vector boson propagators, the general 2-Higgs doublet model, and the minimal supersymmetric standard model in the limit of heavy susy particles) constraints on the models are derived and discussed. In particular we determine upper bounds on the top quark mass in the considered classes of generalized electroweak models. We find that due to the restrictions of the Zbb vertex the top quark mass cannot be higher than in the minimal model.