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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 $E_6$ and $SO(10)$ models. We concentrate on $i$) the effects of the mixing of new neutral gauge bosons with the standard $Z_0$; $ii$) 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 $Z_0$--$Z_1$ mixing, which are always $\lsim 0.01$ and are at the level of a few {\it 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 $M_{Z'} > 1\,$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.