Observable Physics from Superstring Exotic Particles: Small Dirac Neutrino Masses

Certain superstring theories contain new exotic particles and couplings active at low energies. We show that they offer a solution to a long-standing problem of simultaneously having naturally small Dirac neutrino masses and vanishing Majorana masses. It is therefore consistent with (a) lack of observation of neutrinoless double $\beta$ decay, (b) possible evidence for nonvanishing neutrino mass, and (c) a recently proposed solution to the solar neutrino problem.