SEARCH FOR UNSTABLE HEAVY NEUTRAL LEPTONS IN E + E- ANNIHILATIONS AT SQUARE-ROOT-S FROM 50 TO 60.8 GEV

A search for unstable heavy neutral leptons has been made at center-of-mass energies from 50 to 60.8 GeV with the AMY detector at the KEK Storage ring TRISTAN. The neutral leptons are assumed to decay via mixing to electrons and muons. Events with two leptons were searched for. No evidence for their existence was found. Limits in the mass range \ensuremath{\le}28.1 GeV/${c}^{2}$ and mixing-parameter range 9\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}10}$\ensuremath{\le}\ensuremath{\Vert}${U}_{\mathrm{lL}}$${\ensuremath{\Vert}}^{2}$\ensuremath{\le}1 are presented for Dirac- and Majorana-type neutrinos.