RESULTS FOR PRECISION OBSERVABLES IN THE ELECTROWEAK STANDARD MODEL AT 2-LOOP ORDER AND BEYOND

Higher-order contributions to the precision observables Delta r, sin(2 ) theta(eff) and Gamma(l) are discussed. The Higgs-mass dependence of the observables is investigated at the two-loop level, and exact resul ts are derived for the Higgs-dependent two-loop corrections associated with fermions. The top-quark corrections are compared with the result s obtained by an expansion in the top-quark mass up to next-to-leading order. For the pure fermion-loop contributions to Delta r results up to four-loop order are derived. They allow to investigate the validity of the commonly used resummation of the leading fermionic contributio ns to Delta r.