Tests of the Standard Model at LEP

Measurements made by the four LEP experiments of electroweak parameters and the strong couphng constant are presented. These are used in an overaU Standard Model fit and a constraint on the mass of the top quark is derived. 1. Precision Measurements of Electroweak Parameters During 1989, 1990 and 1991 the four main experiments at the LEP e+ecollider (ALEPH, DELPHI, L3 and OPAL) have recorded data representing a total of approximately 1.8 x 106 multihadronic decays of the Z“. These data have been used to measure many parameters of the electroweak sector of the Standard Model. The results presented in this review are preliminary and give the status of the analyses at the time of the 1992 SLAC Summer Institute. 1.1 LEP Beam Energy Calibration A precise calibration of the LEP beam energy is vital for tde measurement of the mass, Mz, and width, Fz, of the Z“. During 1991 this was done, for the first time at LEP, by tie method of resonant spin depolarisation [l!. When a beam circulates freely in a storage ring it tends to become transversely polarised due to synchrotron radiation. Such transverse polarisation has been measured during single-beam operation at LEP several times at the level of 5-16Y0. This can be exploited to calibrate the beam energy by using an oscillating magnetic field to induce a resonant depolarisation. The frequency of the magnetic field is scanned and depolarisation occurs when it matches the spin precession:frequency of the circulating electrons.