Search for neutral heavy leptons produced in Z decays

Weak isosinglet Neutral Heavy Leptons (nu(m)) have been searched for using data collected by the DELPHI detector corresponding to 3.3 x 10(6) hadronic Z(0) decays at LEP1. Four separate searches have been performed, for short-lived nu(m) production giving monojet or acollinear jet topologies, and for long-lived nu(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(Z(0) --> nu(m)<(nu)over bar>) of about 1.3 x 10(-6) at 95% confidence level for nu(m) masses between 3.5 and 50 GeV/c(2). Outside this range the limit weakens rapidly with the nu(m) mass. The results are also interpreted in terms of Limits for the single production of excited neutrinos.