Particle dichotomy and left-right decomposition ofE6superstring models

In supersymmetric ${\mathrm{E}}_6$ gauge models based on superstring theory, an exactly conserved multiplicative quantum number can be defined to separate all particles into two groups. This dichotomy is especially important if left-right symmetry is realized at low energies. It makes possible a truly massless Dirac neutrino whose right-handed component is effectively inert and a second W boson with a mass perhaps as low as 300 GeV in a model with unconventional particle assignments.