Constraints on physics in and beyond the Minimal Standard Model from LEP experiments

The results of fits to the LEP data, taken up to the end of 1990, on cross-sections for hadron and lepton final states and lepton forward-backward charge asymmetries are presented. The fits are first of all performed to extract the parameters of the improved Born approximation. Within the context of the Minimal Standard Model (MSM), the constraints on ~s, mt and mn from these LEP data, and with the inclusion of other LEP and electroweak data, are examined. The results favour a top-quark mass in the region of about 140 GeV, and a relatively light Higgs mass. However, the upper limit on m~ from such fits is sensitive to the value of the strong coupling constant ~s(Mz)~s. Some differences between the data and the MSM are observed, although these are not yet at a level of significance to indicate physics beyond the MSM. Lower limits on the mass of a possible second Z ~ are examined. Results are then presented in terms of the variables S, T and U. These variables provide a general framework for the description of electroweak radiative effects, and are thus suitable for the investigation of physics beyond the MSM. The variables are defined such that S = T= U = 0 corresponds to the MSM prediction for mt=140GeV and m~t = 300 GeV. A fit to the LEP data on cross-sections for hadron and lepton final states and lepton forward-backward charge asymmetries gives S=-0.44+_0.91 and T=-0.15+0.52. With the addition to the fit of other LEP and electroweak data, the values obtained are S= -1.03_+0.66, T= -0.46_+0.41 and U= -0.13+0.94. These results are only just compatible with the assumed central MSM values S = T = U = O . Some constraints from these results on models beyond the MSM are discussed.