Mixing and Radiative Corrections at LEP I
We present a method for a common treatment of Z ′ exchange, QED corrections, and weak loops in e + e − annihilation. QED corrections are taken into account by convoluting a hard-scattering cross section containing γ, Z, and Z ′ exchange. Weak corrections and ZZ ′ mixing are treated simultaneously by a generalization of weak form factors. Using the properly extended Standard Model program for the Z line shape, ZF I T T ER , we perform and compare two different analyses of the 1990 LEP I data in terms of theories based on the E 6-group and in terms of LR-symmetric models. From the LEP I data alone, the ZZ ′ mixing angle may be limited to |θ M | ≤ 0.01 and the Z ′ mass to M 2 > 118–148 GeV, depending on the model (95% CL).
Physics of precision experiments with Z's
Abstract We review the status of precision tests of the electroweak Standard Model. Radiative corrections play an important role for the interpretation of precision measurements and provide a window for the observation of new physics . Implications of recent results from LEP and hadron colliders are discussed. So far the agreement between experimental data and standard model predictions is almost perfect. In order to establish possible deviations from the Standard Model we have to wait for the next step in accuracy. This will be achieved during this year with about 10 6 Z's per experiment.
Measurement of Z0 decays to hadrons, and a precise determination of the number of neutrino species
Abstract We have made a precise measurement of the cross section for e+e−→Z0→hadrons with the L3 detector at LEP, covering the s range from 88.28 to 95.04 GeV. From a fit to the Z0 mass, total width, and the hadronic cross section to be MZ0=91.160 ± 0.024 (experiment) ±0.030(LEP) GeV, ΓZ0=2.539±0.054 GeV, and σh(MZ0)=29.5±0.7 nb. We also used the fit to the Z0 peak cross section and the width todetermine Γinvisible=0.548±0.029 GeV, which corresponds to 3.29±0.17 species of light neutrinos. The possibility of four or more neutrino flavors is thus ruled out at the 4σ confidence level.