Neutrino oscillations induced by two-loop radiative mechanism

Two-loop radiative mechanism, when combined with an U(1)L′ symmetry generated by Le − Lμ − Lτ (=L′), is shown to provide an estimate of Δm2⊙/Δm2atm ∼ ϵme/mτ, where ϵ measures the U(1)L′-breaking. Since Δm2atm ∼ 3.5×10−3 eV2, we find that Δm2⊙ ∼ ϵ10−6 eV2, which will fall into the allowed region of the LOW solution to the solar neutrino problem for ϵ ∼ 0.1.