Light Dirac neutrinos in left-right symmetric and horizontal gauge models

We describe a scheme to generate light Dirac neutrinos. It uses a heavy symmetry-breaking scale to avoid introducing very small couplings or vacuum expectation values. We show how the scheme can be applied to left-right symmetric and to horizontal gauge models with four generations of fermions. In these models neutrino oscillations occur only between pairs of generations (e.g. νsub(e)-νsub(tau) oscillations can occur with no νsub(μ)-νsub(e) or νsub(μ)-νsub(tau) oscillations). (orig.)