Abstract
The dominant electroweak two-loop corrections to the precision observables
M_W and s_w^eff are calculated in the MSSM. They are obtained by evaluating the
two-loop Yukawa contributions of O(alpha_t^2), O(alpha_t alpha_b), O(alpha_b^2)
to the quantity Delta rho. The 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
supersymmetry is given, and two different renormalization schemes are applied.
Compared to the previously known result for the quark-loop contribution we find
a shift of up to +8 MeV in M_W and -4 * 10^-5 in s_w^eff. Detailed numerical
estimates of the remaining uncertainties of M_W and s_w^eff from unknown
higher-order contributions are obtained for different values of the
supersymmetric mass scale.
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