Low-Scale See-Saw Mechanisms for Light Neutrinos
Francesca Borzumati, Yasunori Nomura
TL;DR
This work develops two complementary SUSY-based routes to generate light neutrino masses with three right-handed neutrinos, relaxing the standard high-scale seesaw requirement. The first route suppresses tree-level Yukawas via a horizontal symmetry and a spurion field Z, yielding naturally small Dirac masses and, depending on charges, either Dirac or Majorana components. The second route leverages radiative generation through sneutrino-neutralino loops in a theory with gauged U(1)_{B-L}, producing Dirac and Majorana masses with suppression factors set by the mass hierarchy in the neutralino and sneutrino sectors, allowing a separation between the right-handed-scale $M_R$ and the B-L breaking scale $M_G$. The paper provides explicit formulas for $m_D$ and $m_L$ in various regimes of $M_R$, outlines an explicit model embedding within an IYIT-like SUSY framework, and discusses the resulting neutrino spectra, including scenarios with light sterile states and Dirac-only masses. Collectively, these mechanisms broaden the landscape of realistic neutrino mass models and suggest that low-scale (sub-TeV to TeV) B-L breaking scales can be phenomenologically viable, with distinctive implications for neutrino oscillations, collider signals, and cosmology.
Abstract
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 M_G is introduced, a wider range of possibilities opens up. The value of the right-handed neutrino mass M_R can be easily disentangled from that of M_G. Dirac and Majorana neutrino masses at the eV scale can be generated radiatively through the exchange of sneutrinos and neutralinos. Dirac masses m_D owe their smallness to the pattern of light-heavy scales in the neutralino mass matrix. The smallness of the Majorana masses m_L 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 M_R, 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 m_D/M_R, or one light Dirac state. In the second scenario, with a large value of M_R, the physical eigenstates are two nearly unmixed Majorana states with masses \sim m_L and \sim M_R. In both cases, the (B-L)-breaking scale M_G is, in general, much smaller than that in the traditional see-saw mechanism.