Search for neutral heavy leptons produced in Z decays

Weak isosinglet Neutral Heavy Leptons (v _m) have been searched for using data collected by the DELPHI detector corresponding to 3.3 × 10⁶ hadronic Z⁰ decays at LEP1. Four separate searches have been performed, for short-lived v _m production giving monojet or acollinear jet topologies, and for long-lived v _m giving detectable secondary vertices or calorimeter clusters. No indication of the existence of these particles has been found, leading to an upper limit for the branching ratio BR(Z⁰ → v _m ?) of about 1.3 × 10^?6 at 95% confidence level for v _m masses between 3.5 and 50 GeV/c². Outside this range the limit weakens rapidly with the v _m mass. The results are also interpreted in terms of limits for the single production of excited neutrinos.