CONSTRAINTS FROM LEPTON UNIVERSALITY AT THE Z-PEAK ON UNIFIED THEORIES

We suggest the use of a Universality-breaking observable based on lept on asymmetries as derived from the left-right asymmetry and the tau po larization at the Z peak, which can efficiently constrain the paramete r space of unified theories. The new observable is complementary to th e leptonic partial width differences and it depends critically on the chirality of a possible non-universal Z-boson coupling to like-flavour leptons. The LEP/SLC potential of probing universality violation is d iscussed in representative low-energy extensions of the Standard Model (SM) that could be derived by supersymmetric grand unified theories, such as the SM with left-handed and/or right-handed neutral isosinglet s, the left-right symmetric model, and the minimal supersymmetric SM.