Γ(Z → banti b): A signature of hard mass terms for a heavy top

We calculate analytically the weak radiative corrections to the weak neutral current gauge boson-bottom fermion vertex, keeping the mass mt of the internal fermion line for the relevant diagrams. We find, to order α, a hard mass-term dependence mt2/MW2 of the amplitude, for large mt values. Its origin comes from the unphysical charged Higgs coupling to fermions in the renormalizable gauge or, equivalently, from the longitudinal charged gauge boson couplings. The diagonal Z0 decay width to b-quarks decreases, due to these weak radiative corrections, by 0.6%-2.5% when the top mass mt varies from 45 to 200 GeV. (orig.)