The Z boson, the top quark, and non-minimal Higgs bosons

The impacts of a heavy top quark on the relation between the vector boson masses and the mixing angle at the Z peak, the on-resonance asymmetries, and the Z decay widths are reviewed in the minimal model and extensions within SU(2) × U(1). Of particular interest are models with two scalar doublets where the charged Higgs bosons can give a sizeable negative contribution to the partial width This enhancement of the Standard Model top effect can be used to separate models with charged Higgs bosons coupling to the (t, b) doublet from other classes of models affecting only the ρ-parameter. Precision measurements of the Z partial widths will provide significant constraints on the two doublet models if the top quark is very heavy.