Phenomenology of Massive Vectorlike Doublet Leptons

Massive vectorlike electroweak doublets are generic in many extensions of the standard model. Even though one member of the doublet is necessarily electrically charged these particles are not easily detected in collider experiments. The neutral and charged states within the doublet are split by electroweak symmetry breaking. In the absence of mixing with other states, the radiatively generated splitting is in the range 200--350MeV for m1/2mZ. The charged state decays to the neutral one with an O(cm) decay length, predominantly by emission of a soft charged pion. The best possibility to detect these massive charged particles is to trigger on hard initial state radiation and search for two associated soft charged pions with displaced vertices. The mass reach for this process at LEPII is limited by luminosity rather than kinematics.