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 ${\gamma }_5$ preserves the latter.

• Received 3 November 1994

DOI:https://doi.org/10.1103/PhysRevD.51.3820