Universality-Breaking Effects in Leptonic Z Decays

We analyze the possibility of universality violation in diagonal leptonic decays of the $Z$ boson, in the context of interfamily "see-saw" models. In a minimal extension of the Standard Model with right-handed neutrino fields, we find that universality-breaking effects increase quadratically with the heavy Majorana neutrino mass and may be observed in the running $LEP$ experiments.