Heavy and Excited Leptons in the OPAL Detector?
  This M.Sc. thesis describes a search for exotic leptons. The search has been performed using data from the OPAL detector at the Large Electron Positron collider at CERN. The total integrated luminosity was 663 pb$^{-1}$ with center of mass energies in the range of 183-209 GeV. The work has been concentrated on the study of production of heavy leptons via the charged current channel and disintegration of the $W$ into two quarks. In particular, single production of heavy leptons in the mass region 100-170 GeV has been extensively studied. No evidence for any new particles has been found. The results translate into upper limits on the mixing between the heavy and the ordinary lepton for different heavy lepton masses. The limits on the mixing angles are generally improved in comparison with the nominal value $\zeta^2\sim 0.005$. 
Erik Elfgren