MU-E CONVERSION IN NUCLEI AND Z' PHYSICS

Together with the existence of new neutral gauge bosons, models based on extended gauge groups (rank > 4) often predict also new charged fer mions. A mixing of the known fermions with new states with exotic weak -isospin assignments (left-handed singlets and right-handed doublets) will induce tree level flavour changing neutral interactions mediated by Z exchange, while if the mixing is only with new states with ordina ry weak-isospin assignments, the flavour changing neutral currents are mainly due to the exchange of the lightest new neutral gauge boson Z' . We show that the present experimental limits on mu-e conversion in n uclei give a nuclear-model-independent bound on the Z-e-mu vertex whic h is twice as strong as that obtained from mu --> eee. In the case of E6 models these limits provide quite stringent constraints on the Z' m ass and on the Z-Z' mixing angle. We point out that the proposed exper iments to search for mu-e conversion in nuclei have good chances to fi nd evidence of lepton flavour violation, either in the case that new e xotic fermions are present at the electroweak scale, or if a new neutr al gauge boson Z' of E6 origin lighter than a few TeV exists.