The neutral fermion sectors of E_6-inspired low energy models, in particular
the Alternative Left-Right and Inert models, are considered in detail within
the non-supersymmetric scenario. We show that in their simplest form, these
models always predict, for each generation, the lightest neutrino to be an
SU(2)_L singlet, as well as two extra neutrinos with masses of the order of the
up-quark mass. In order to recover Standard Model phenomenology, additional
assumptions in the form of discrete symmetries and/or new interactions are
needed. These are classified as the Discrete Symmetry (DS), Higher Dimensional
Operators (HDO), and Additional Neutral Fermion (ANF) methods. The DS method
can solve the problem, but requires additional Higgs doublets that do not get
vacuum expectation values. The HDO method predicts no sterile neutrino, and
that the active neutrinos mix with a heavy isodoublet neutrino, thus slightly
suppressing the couplings of active neutrinos, with interesting
phenomenological implications. The ANF method also predicts this suppression,
and also naturally includes one or more "sterile" neutrinos. This scenario
allows the existence of sterile neutrino(s) in either a 3+1 or 2+2 structure at
low energies, which are favored by the LSND result.Comment: 25 pages, 1 figure; typos corrected, version to be published in
Physical Review
