Two-Loop Radiative Corrections of Electroweak Mixing Angle and Branching Fraction for Z->bb.

In this thesis I develop a numerical technique which is based on the Mellin-Barnes representation to calculate two-loop Feynman integrals. The resulting complex integrals of high dimensions are being applied with some treatments, such as the variable transform, reduction formulas, etc, to improve the convergence of the integrals. The approach is adopted to compute the two-loop radiative corrections of the electroweak mixing angle and the hadronic branching ratio for the process Z->bb. I focus on contributions with an internal fermion sub-loop using the on-shell renormalization scheme. The results will help to derive improved constraints on the Higgs particle