Search for excited leptons in $e^+ e^-$ collisions at $\sqrt{s}$ =161 GeV

A search with the DELPHI detector at LEP for excited leptons ($e^*$, $\mu^*$, $\tau^*$ and $\nu^*$) decaying through $\gamma$, W or Z transitions is reported. The data used correspond to an integrated luminosity of 10 pb$^{-1}$ at an $e^+e^-$ centre-of-mass energy of 161 GeV. The search for pair produced excited leptons established the limits $m_{e^*} > 79.6 $ GeV/$c^2$, $m_{\mu^*} > 79.6$ GeV/$c^2$, $m_{\tau^*} > 79.4$ GeV/$c^2$ and $m_{\nu^*} > 56.4$ GeV/$c^2$ at 95\% confidence level, assuming that the SU(2) and U(1) couplings of excited leptons are in the same ratio as for normal leptons and considering both left and right-handed components. Limits corresponding to other assumptions are also given. The search for single excited lepton production established upper limits on the ratio $\lambda/m_{\ell^*}$ of the coupling of the excited lepton to its mass.