Search for neutral heavy leptons in DELPHI

Weak isosinglet neutral heavy leptons ( nu /sub m/) has been searched for in leptonic decays of hadrons and in neutrino beam experiments, leading to stringent upper limits on U /sup 2/ reaching 10/sup -7/ in the low mass region (m/sub nu -m/ below 2-3 GeV/c/sup 2/). OPAL and L3 have also placed limits of the order 10/sup -4/ to 10/sup -3/ for masses above 3 GeV/c/sup 2/. A search for isosinglet neutral heavy leptons ( nu /sub m/) has been carried out using data collected by the DELPHI detector corresponding to 3.3*10/sup 6/ hadronic Z/sup 0/ decays at center of mass energy of square root s~m(Z/sup 0/). The data and experimental techniques used in DELPHI improve on the previous sensitivities in the large mass range by about one order of magnitude. Special emphasis has been given to the search for monojet- like topologies for which one event has passed the selection, in agreement with the expectation from the reaction: e/sup +/e/sup -/ to ll nu nu . These results are also interpreted in terms of limits for the single production of excited neutrinos. (9 refs).