Search for unstable neutral and charged heavy leptons in e+e− collisions at s=164 GeV

Searches for unstable neutral and charged heavy leptons (L0, L±) have been performed using a data sample of 10.0 pb−1 at a centre-of-mass energy of s=161.3 GeV collected with the OPAL detector at LEP during July and August 1996. Two candidate events were observed after the selection, which is consistent with the expected total background of 1.57 events, and lower limits have been derived on heavy lepton masses for various models. If an unstable Dirac neutral heavy lepton, L0, decays only into eW∗, μW∗ or τW∗, the lower limits on its mass at the 95% C.L. are 69.3 GeV, 72.0 GeV and 66.0 GeV, respectively. The limits are modified for a Majorana L0 to 59.5 GeV, 60.5 GeV and 55.7 GeV, respectively. If an unstable heavy charged lepton, L±, decays into a stable heavy neutrino, and if ml± - mνl > 13 GeV, a lower limit on its mass of 73.5 GeV is obtained. If the L± decays through lepton flavour mixing into a massless neutrino νi and W∗±, the lower limit on ml± was determined to be 76.7 GeV at 95% C.L.