Gravity-assisted neutrino masses

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 $G_{\mathrm{res}}$ that forbids light-neutrino masses. The observed light-neutrino masses then arise because gravity breaks $G_{\mathrm{res}}$ 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 $\mathcal{O}(1)$ operator coefficients, it predicts a region in the ($M_R$, $\Theta^2$)-plane that can be testable via displaced-vertex searches at the High-Luminosity (HL) LHC and the FCC-ee.