CONSTRAINTS FROM LEPTON UNIVERSALITY AT THE Z PEAK ON UNIFIED THEORIES

We suggest the use of a universality-breaking observable based on lepton asymmetries as derived from the left-right asymmetry and the $τ$ polarization at the $Z$ peak, which can efficiently constrain the parameter space of unified theories. The new observable is complementary to the 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 discuss...