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.

• Received 25 June 1993

DOI:https://doi.org/10.1103/PhysRevLett.71.2695