Leading Electroweak Two-Loop Corrections to Precision Observables in the MSSM

The leading electroweak MSSM two-loop corrections to the electroweakprecision observables are calculated. They are obtained by evaluating thetwo-loop O(\alpha_t^2), O(\alpha_t \alpha_b), O(\alpha_b^2) contributions tothe quantity \Delta\rho in the limit of heavy scalar quarks, i.e. we considerthe contributions of a Two-Higgs-Doublet model with MSSM restrictions. The fullanalytic result for arbitrary values of the lightest CP-even Higgs boson massis presented. The numerical effects of the leading electroweak MSSM two-loopcorrections on the precision observables M_W and sin^2(theta_eff) are analyzed.The electroweak two-loop contribution to M_W amounts up to -12 MeV and up to +610^-5 for sin^2(theta_eff). The corrections from the bottom quark loops canbecome important for large values of tan beta. They enter with a different signthan the O(\alpha_t^2) corrections. We furthermore investigate the currentsensitivity of the electroweak precision observables to the top Yukawa couplingin the SM and the MSSM. The prospects for indirectly determining this couplingat the next generation of colliders are discussed.Comment: 33 pages, 10 figure