Two-loop large top mass corrections to electroweak parameters: Analytic results valid for arbitrary Higgs boson mass

We present analytic results for the two-loop leading heavy top-quark contributions as required for precise predictions of electroweak observables. The Higgs boson mass is kept arbitrary. We discuss a set of recurrence relations for diagrams with three masses at zero momentum, which allow one to reduce the calculation to a few master integrals. Simple analytic expressions are obtained for the {ρ} parameter and for the Zbb{} vertex. Results are pesented for the on-shell and the MS{} definition of the top quark mass. We explicitly discuss and verify the validity of Ward-Takahashi identities and check that the use of an anticommuting γ₅ preserves the latter.