New Neutral Gauge Bosons and New Heavy Fermions in the Light of the New LEP Data

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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' > 1TeV. 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.