TTP99-10 Non-Singlet Corrections of $O(\alpha_s^2 G_F M_t^2)$ to $\Gamma(Z \to \bar{b} b)$

The partial decay rate of the $Z$ boson into bottom quarks constitutes an important decay channel. This is mainly due to the virtual presence of the top quark in the loop diagrams giving rise to correction factors which are quadratic in the top quark mass. At one- and two-loop order it turned out that the leading term in the heavy-top expansion leads to very good approximations to the exact result. In this work the non-singlet diagrams at O(\alpha_s^2 G_F M_t^2) are considered.