Experimental and theoretical implications of new sequential leptons

If new sequential leptons $E^{\pm }$ and $N^0$ exist, the CERN LEP bound implies $m_E$, $m_N>\frac{M_Z}{2}$. The heaviness of the neutral lepton breaks away from the pattern of the first three generations. The minimal model is to have 4 left-handed lepton doublets and 4 right-handed charged lepton singlets, but only one right-handed neutral lepton singlet. Since in general the third and fourth generations should mix, and since $|m_N-m_E|$ should not be too large, neither $E$ nor $N$ would be stable, and both tend to decay via the Cabibbo suppressed $E\to {\nu }_{\tau }$ or $N\to \tau$ charged currents. This leads to the interesting signature of like-sign $W$ pair production via $E^{+}N\to {\overline{\nu }}_{\tau }{\tau }^{-}{W}^{+}{W}^{-}$ at the SSC and CERN LHC. The popular seesaw mechanism cannot plausibly accommodate the near masslessness of the light neutrinos and the heaviness of $N^0$ simultaneously. The representation structure poses a difficulty to the traditional approach of SO(10)-based grand unified theories. The discovery of such new heavy leptons would thus have interesting implications.