Search for unstable heavy and excited leptons at LEP2

Abstract

Searches for unstable neutral and charged heavy leptons, N and ${\rm L^\pm}$ , and for excited states of neutral and charged leptons, $\nu^*$ , ${\rm e}^*$ , $\mu^*$ , and $\tau^*$ , have been performed in ${\rm e^+e^-}$ collisions using data collected by the OPAL detector at LEP. The data analysed correspond to an integrated luminosity of about 58 pb $^{-1}$ at a centre-of-mass energy of 183 GeV, and about 10 pb $^{-1}$ each at 161 GeV and 172 GeV. No evidence for new particles was found. Lower limits on the masses of unstable heavy and excited leptons are derived. From the analysis of charged-current, neutral-current, and photonic decays of singly produced excited leptons, upper limits are determined for the ratio of the coupling to the compositeness scale, $f/\Lambda$ , for masses up to the kinematic limit. For excited leptons, the limits are established independently of the relative values of the coupling constants f and $f^\prime$ .