Light Dirac neutrino in left-right-symmetric models

Recent experiments related to neutrino mass and neutrinoless double-{β} decay suggest that the electron neutrino is a Dirac particle with its mass of the order of 10--20 eV. In most of the existing grand-unified scenarios neutrinos turn out to be Majorana particles, unless an arbitrarily small Yukawa coupling and/or Higgs-field vacuum expectation value is introduced. In this paper we propose a model where one can readily accommodate an ultralight Dirac neutrino. It has been pointed out that any left-right-symmetric model with a second set of heavier fermions can have an ultralight Dirac neutrino, if the global symmetry which prevents any mixing between the two sets of fermions allows a mixing between antifermions and the heavier fermion. The complete potential has been minimized to obtain the mass matrix. It is shown that the required symmetry-breaking mass scales for the present case are consistent with low-energy values of sin? sup 2 theta sub w--- and αₛ.