Neutral Heavy Leptons (NHLs) arise in many extensions of the standard electroweak theory such as superstring inspired models. The possibility of gauge singlets NHLs is especially attractive because it gives an explanation for the observed smallness of the neutrino mass. Existing limits on the possible existence of such particles are still fairly poor. We have investigated isosinglet NHL production and decays within different models. The dominant production cross section is single production (i.e. Z0 → N + overlineνor Z0 → overlineN + ν ) as a result of mixing with the standard doublet neutrinos. Subsequent NHL decays lead to striking signatures. Taking into account the expected luminosities and typical detector efficiencies of the different LEP/SLC experiments we conclude that these may discover isosinglet NHLs or else substantially improve and extend present limits on their mass and co... [Llegir més ...]