New neutral gauge bosons and new heavy fermions in the light of the new LEP data

We derive limits on a class of new physics effects that are naturally present in grand unified theories based on extended gauge groups, and in particular in E6 and SO(10) models. We concentrate on i) the effects of the mixing of new neutral gauge bosons with the standard Z0; ii) the effects of a mixing of the known fermions with new heavy states. We perform a global analysis including all the LEP data on the Z decay widths and asymmetries collected until 1993, the SLC measurement of the left-right asymmetry, the measurement of the W boson mass, various charged current constraints, and the low energy neutral current experiments. We use a top mass value in the range announced by CDF. We derive limits on the Z0-Z1 mixing, which are always ≲ 0.01 and are at the level of a few per mille if some specific model is assumed. Model-dependent theoretical relations between the mixing and the mass of the new gauge boson in most cases require MZ′ > 1 TeV. Limits on light-heavy fermion mixings are also largely improved with respect to previous analyses, and are particularly relevant for a class of models that we discuss.