Low-Scale See-Saw Mechanisms for Light Neutrinos

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Alternatives to the see-saw mechanism are explored in supersymmetric models with three right-handed or sterile neutrinos. Tree-level Yukawa couplings can be drastically suppressed in a natural way to give sub-eV Dirac neutrino masses. If, in addition, a B-L gauge symmetry broken at a large scale MG is introduced, a wider range of possibilities opens up. The value of the right-handed neutrino mass Mᵣ can be easily disentangled from that of MG. Dirac and Majorana neutrino masses at the eV scale can be generated radiatively through the exchange of sneutrinos and neutralinos. Dirac masses mD owe their smallness to the pattern of light-heavy scales in the neutralino mass matrix. The smallness of the Majorana masses mₗ is linked to a similar see-saw pattern in the sneutrino mass matrix. Two distinct scenarios emerge. In the first, with very small or vanishing Mᵣ, the physical neutrino eigenstates are, for each generation, either two light Majorana states with mixing angle ranging from very small to maximal, depending on the ratio mD/Mᵣ, or one light Dirac state. In the second scenario, with a large value of Mᵣ, the physical eigenstates are two nearly unmixed Majorana states with masses mₗ and Mᵣ. In both cases, the (B-L)-breaking scale MG is, in general, much smaller than that in the traditional see-saw mechanism.