Higher-order supersymmetric contributions to electroweak precision observables

The dominant electroweak two-loop corrections to the precision observables M(_w) and sin(^2) θ off are calculated in the MSSM. They are obtained by evaluating the two loop Yukawa contributions of ϭ (aas) and ϭ (at2), ϭ (a2ab), ϭ (a2b), to the quantity Ap. A review of the one-loop Standard Model calculation is given in the large Top-Yukawa coupling limit. The 0{aļ), 0{atab), 0{al) result, involving the contributions from Standard Model fermions, sfermions, Higgs bosons and higgsinos, is derived in the gauge- less limit for arbitrary values of the lightest CP-even Higgs boson mass. A thorough discussion of the parameter relations enforced by super symmetry is given. Two different renormalisation schemes are applied. Compared to the previously known result for the quark-loop contribution we find a shift of up to +8 MeV in Mw and —4 X 10—5 in sin2 ^eff- Detailed numerical estimates of the remaining uncertainties of Mw and sin2 もff from unknown higher-order contributions are obtained for different values of the supersymmetric mass scale. The calculations are preceded by a review of EWPO and super symmetry. The electroweak precision variable Ap is defined. We renormalise using both dimensional regularisation and dimensional reduction. [brace not closed]