SLC/LEP CONSTRAINTS ON UNIFIED MODELS

We examine the potential of constraining possible nondecoupling effects of heavy neutrinos and Higgs bosons at LEP and SLC that may show up in the nonoblique part of the $Zl_il_j$ couplings. We analyze this type of new-physics interactions within the context of low-energy scenarios motivated by unified theories, such as the Standard Model (SM) with neutral isosinglets, the left-right symmetric model, and the minimal supersymmetric SM. Our analysis comprises a complete set of physical quantities based on the nonobservation of flavour-violating $Z$-boson decays, lepton universality in the decays $Z\to l\bar{l}$, and universality of lepton asymmetries at the $Z$ peak. It is found that these quantities form a set of complementary observables and may hence constrain the parameter space of the theories. Non-SM contributions of new-physics interactions to $R_b=\Gamma(Z\to b\bar{b})/\Gamma(Z\to \mbox{hadrons})$ are briefly discussed within these models.