Global Fits of the SM and MSSM to Electroweak Precision Data

A program including all radiative corrections to the MSSM at the same level as the radiative corrections to the SM has been developed and used to perform global fits to all electroweak data from LEP, SLC and the Tevatron and the radiative b->sgamma decay from CLEO. Values of the strong coupling constant at the $M_Z$ scale and $\sin^2\theta_{\overline{MS}}$ are derived, both in the SM and MSSM. Recent updates on electroweak data, which have been presented at the Warsaw Conference in summer 1996, reduce the R_b anomaly from a 3.2 sigma to a 1.8 sigma effect. In addition, the $b \to s\gamma$ decay is 30% below the SM prediction. In the MSSM light stops and light charginos increase $R_b$ and decrease the $b \to s\gamma$ rate, so both observations can be brought into agreement with the MSSM for the same region of parameter space. However, the resulting $\chi^2$ value for the MSSM fits is only marginally lower and in addition, the splitting in the stop sector has to be unnaturally high.