Updated Global Fits of the SM and MSSM to Electroweak Precision Data

The Minimal supersymmetric extension of the Standard Model (MSSM) with light stops, charginos or pseudoscalar Higgs bosons has been suggested as an explanation of the too high value of the branching ratio of the $Z^0$ boson into $b$ quarks ($R_b$ anomaly). 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. 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, which improves the probability in the SM from 8% to 24%. Although the MSSM can fully explain the remaining deviation of R_b from the SM value, its probability is not better because of the larger number of free parameters. The best fitted value of the strong coupling constant at the M_Z scale in the {MS} renormalization scheme is 0.120\pm0.003 for the SM and 0.116\pm0.005 (0.119\pm0.005) for the MSSM with low (high) values of tan beta. The derived quantity \sin^2\theta_{MS} equals 0.2317\pm0.0004 (0.2315\pm0.0004) for the SM (MSSM).