Present evidences for neutrino masses and lepton flavour mixings allow to predict, in the Standard Model with light neutrinos, branching rates for the decays Z --> e mu, mu tau, e tau of less than 10^{ -54}, while present experimental exclusion limits from LEP 1 are of order 10^{ -5}. The GigaZ option of the TESLA Linear Collider project will extend the sensitivity down to about 10^{ -8}. We study in a systematic way some minimal extensions of the Standard Model and show that GigaZ might well be sensitive to the rates predicted from these scenarios.
Categories: hep-ph