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Abstract |
Gravity is generally expected to violate global symmetries, including lepton number. However, neutrino masses from the Planck-suppressed Weinberg operator are typically too small to account for oscillation data. We propose a new model-building approach to low-scale neutrino mass generation, in which an intermediate spontaneous symmetry-breaking scale generates masses and mixings in the heavy neutral lepton (HNL) sector, while leaving an unbroken residual symmetry Gres that forbids light-neutrino masses. The observed light-neutrino masses then arise because gravity breaks Gres via Planck-suppressed operators, inducing the small lepton-number violation required in low-scale seesaw constructions. The HNLs form pseudo-Dirac pairs, with masses potentially within reach of future colliders and complementary tests in precision searches such as charged lepton flavour violation (cLFV). As an illustration, we present a representative realisation of this class of models and show that, for O(1) operator coefficients, it predicts a region in the (MR, Theta 2)-plane that can be testable via displaced-vertex searches at the High-Luminosity (HL-)LHC and the FCC-ee. |
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