Exact nonfactorizable O(alpha(8)g(2)) two-loop contribution TO Z -> b(b)over-bar

For Z --> bb, we calculate all the two-loop top-dependent Feynman graphs, w hich have mixed QCD and electroweak contributions that are not factorizable . To evaluate the graphs, without resorting to a mass expansion, we apply a two-loop extension of the one-loop Passarino-Veltman reduction. This is an analytic-numerical method, which first converts all diagrams into a set of ten standard scalar functions, and then integrates them numerically over t he remaining Feynman parameters, with rapid convergence and high accuracy. We discuss the treatment of ir singularities within our methods. We do not resort to unitarity cuts of two-point functions for calculating decay fates ; these are useful only to obtain an inclusive rate. For this reason, exper imental cuts and the experimental ir energy resolution can be implemented i n our calculation, once the corresponding one-loop gluon bremsstrahlung pro cess is added to this calculation.