UM-TH 93 – 08 FTUV 93 – 14 μ-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 fermions. 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 ordinary 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 μ − e conversion in nuclei give a nuclear-modelindependent bound on the Z-e-μ vertex which is twice as strong as that obtained from μ → eee. In the case of E6 models these limits provide quite stringent constraints on the Z ′ mass and on the Z − Z ′ mixing angle. We point out that the proposed experiments to search for μ−e conversion in nuclei have good chances to find evidence of lepton flavour violation, either in the case that new exotic fermions are present at the electroweak scale, or if a new neutral gauge boson Z ′ of E6 origin lighter than a few TeV exists.