Universality-breaking effects in leptonicZdecays

We analyze the possibility of universality violation in diagonal leptonic decays of the Z boson, in the context of interfamily ‘‘seesaw’’ 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 current experiments at the CERN ${\mathit{e}}^{+}$ ${\mathit{e}}^{\mathrm{-}}$ collider LEP.