Search for isosinglet neutral heavy leptons in Z0 decays

Abstract We search for neutral heavy leptons that are isosinglets under the standard SU (2) L gauge group. Such neutral heavy leptons are expected in many extensions of the standard model. Three types of heavy leptons N e , N μ , N τ associated with the three neutrino types v e , v μ , v τ have been directly searched for and no evidence for a signal has been found. We set the limit Br(N Su 0 → v l N l ) x 10 −5 at the 95% CL for mass range from 3 GeV up to M z .