Experimental and Theoretical Implications of New Sequential Leptons

If new sequential leptons $E^\pm$ and $N^0$ exist, the LEP bound implies $m_E$, $m_N > M_Z/2$. 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 3rd and 4th generation should mix, and since $\vert m_N - m_E\vert$ 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 $E^+N \to \bar\nu_\tau\tau^- W^+W^+$ at the SSC and 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 rather wide ranging and far reaching implications.