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Gravity-assisted neutrino masses

Stefan Antusch, Salvador Centelles Chuliá, Miguel Levy

TL;DR

This work addresses the puzzle of tiny neutrino masses by proposing a gravity-assisted low-scale seesaw mechanism: a intermediate symmetry-breaking sector generates heavy neutral lepton masses and mixings while a residual symmetry protects light neutrino masses; gravity then breaks the residual symmetry via Planck-suppressed operators, inducing the small lepton-number violation needed for light neutrino masses. In a concrete two-HNL model with a global U(1) broken to Z3, the dominant gravity-induced inverse-seesaw contribution yields a correlated region in the heavy-neutrino mass and mixing parameter space, $1\,\text{keV} \lesssim M_R \lesssim 11\,\text{TeV}$ with $\Theta^2$ within reach of future colliders. A benchmark with $(n_1,n_2)=(5,7)$ demonstrates that HL-LHC and FCC-ee could probe displaced-vertex signatures, linking the symmetry-breaking scale $\\langle \phi\\rangle$ to observables. The framework is technically natural and predictive, and it opens avenues for cosmological signatures and extensions to GUTs, leptogenesis, and dark matter, providing a new path to testable neutrino mass generation mechanisms.

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 ($M_R$, $Θ^2$)-plane that can be testable via displaced-vertex searches at the High-Luminosity (HL) LHC and the FCC-ee.

Gravity-assisted neutrino masses

TL;DR

This work addresses the puzzle of tiny neutrino masses by proposing a gravity-assisted low-scale seesaw mechanism: a intermediate symmetry-breaking sector generates heavy neutral lepton masses and mixings while a residual symmetry protects light neutrino masses; gravity then breaks the residual symmetry via Planck-suppressed operators, inducing the small lepton-number violation needed for light neutrino masses. In a concrete two-HNL model with a global U(1) broken to Z3, the dominant gravity-induced inverse-seesaw contribution yields a correlated region in the heavy-neutrino mass and mixing parameter space, with within reach of future colliders. A benchmark with demonstrates that HL-LHC and FCC-ee could probe displaced-vertex signatures, linking the symmetry-breaking scale to observables. The framework is technically natural and predictive, and it opens avenues for cosmological signatures and extensions to GUTs, leptogenesis, and dark matter, providing a new path to testable neutrino mass generation mechanisms.

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 that forbids light-neutrino masses. The observed light-neutrino masses then arise because gravity breaks 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 operator coefficients, it predicts a region in the (, )-plane that can be testable via displaced-vertex searches at the High-Luminosity (HL) LHC and the FCC-ee.

Paper Structure

This paper contains 5 sections, 15 equations, 1 figure, 1 table.

Table of Contents

  1. Introduction
  2. Theoretical framework
  3. A benchmark scenario for the FCC-$\text{ee}$
  4. Conclusions and Outlook
  5. Acknowledgements

Figures (1)

  • Figure 1: Predictions for $\Theta^2(M_R)$ in the case of $(n_1, n_2)=(5,7)$, shown in Eq. \ref{['eq:MRTheta']}, with coefficients set to one. The shaded region on the bottom is the area below the conventional seesaw line $\Theta^2 = m_\nu/M_R$. In solid, we show the current experimental bounds of LHC prompt searches CMS:2018iaf (red), LHC displaced vertex searches CMS:2022fut (dark red), electroweak precision data delAguila:2008pwdeBlas:2013glaAntusch:2014woaBlennow:2016jkn (orange), and in dashed the future sensitivities of HL-LHC displaced vertex searches Drewes:2019fou (dark red), and of the FCC-ee Blondel:2022qqo (green), with most of the data taken from Bolton:2019pcu.