Unconventional superstring-derivedE6models and neutrino phenomenology

Conventional superstring-derived ${\mathrm{E}}_6$ models can accommodate small neutrino masses if a discrete symmetry is imposed which forbids tree-level Dirac neutrino masses but allows for radiative mass generation. Since the only possible symmetries of this kind are known to be generation dependent, we explore the possibility that the three sets of light states in each generation do not have the same assignments with respect to the 27 of ${\mathrm{E}}_6$, leading to nonuniversal gauge interactions under the additional U(1)' factors for the known fermions. We argue that models realizing such a scenario are viable, with their structure being constrained mainly by the requirement of the absence of flavor-changing neutral currents in the Higgs sector. Moreover, in contrast with the standard case, rank 6 models are not disfavored with respect to rank 5. By requiring the number of light neutral states to be minimal, these models have an almost unique pattern of neutrino masses and mixings. We construct a model based on the unconventional assignment scenario in which (with a natural choice of the parameters) $m_{{\nu }_{\tau }}\sim 10$ eV is generated at one loop, $m_{{\nu }_{\mu }}$ is generated at two loops and lies in a range interesting for the solar neutrino problem, and ${\nu }_e$ remains massless. In addition, since baryon and lepton number are conserved, there is no proton decay in the model. In order to illustrate the nonstandard phenomenology implied by our scheme we also discuss a second scenario in which an attempt for solving the solar neutrino puzzle with matter-enhanced oscillations and practically massless neutrinos can be formulated, and in which peculiar effects for the ${\nu }_{\mu }\to {\nu }_{\tau }$ conversion of the upward-going atmospheric neutrinos could arise as well.