Hybrid MSW + VO Solution of the Solar Neutrino Problem in String-Motivated Unified Theories

It is shown that the hybrid MSW + VO solution of the solar neutrino problem, according to which the solar νe undergo matter-enhanced transitions into νμ,τ in the Sun followed by long wave length (∼ 1.5×108 km) νe ↔ νμ,τ oscillations in vacuum between the Sun and the Earth, can occur naturally in string-motivated grand unified theories. We consider the supersymmetric version of a string-type SU(4) ⊗ SU(2)L ⊗ SU(2)R theory with U(1)X family symmetry, which was shown to successfully describe the charged fermion masses and the quark mixing, and extend the earlier fermion mass analysis to the neutrino sector. We show that the four oscillation parameters ∆m31, ∆m21 and sin 2 2θ12, sin 2 2θ13, characterising the combined matter-enhanced transitions and vacuum oscillations of the solar νe, naturally get values in the ranges of the hybrid MSW + VO solutions found recently.