TTP94-14 O(\alpha\alpha_s\ln m_t^2) non-universal Corrections to the Decay Rate \Gamma(Z\to b\bar b)

The partial decay rate $\Gamma(Z\to b\bar{b})$ is significantly influenced by the mass of the top quark due to electroweak radiative corrections. The leading $\sim m_t^2$ and the next-to-leading contribution $\sim \ln m_t^2$ are known to be numerically of similar size. In this work we calculate the QCD corrections to the logarithmic correction using the heavy top mass expansion. The ${\cal O} (\alpha\as \ln m_t^2)$ corrections are of the same order as the QCD corrections to the quadratic top mass term, but of different sign.