ZZ′ 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, ZFITTER, we perform and compare two different analyses of the 1990 LEP I data in terms of theories based on the E6-group and in LR-symmetric models. From the LEP I data alone, the ZZ′ mixing angle may be limited to |θM| ⩽ 0.01 and the Z′ mass to M2 > 118–148 GeV, in dependence of the model (95% CL)