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Dark Transition Magnetic Moments of Majorana Neutrinos Mediated by a Dark Photon

Haohao Zhang

Abstract

Standard Model predictions for Majorana neutrino transition magnetic moments (TMMs) are subject to severe chiral and GIM-like suppressions, rendering them vanishingly small. To dynamically generate a macroscopic TMM, we propose a dark sector framework featuring a $U(1)_D$ gauge symmetry, a vector-like lepton doublet, and two complex dark scalars. We demonstrate that while fermion-radiated loop amplitudes identically cancel due to Majorana self-conjugacy, a chirally enhanced dark TMM is successfully generated exclusively through scalar-radiated loops. This mechanism safely shifts the required chirality flip onto the heavy internal fermion line and utilizes a misaligned double-scalar mixing in flavor space to evade the Majorana antisymmetry prohibition. We systematically confront this tensor portal framework with multi-frontier experimental constraints. Since the dark TMM generation is inextricably linked to charged lepton flavor violation, the internal Yukawa couplings are stringently capped by the latest $μ\to e γ$ limits from MEG II. Concurrently, the visible-dark kinetic mixing portal is heavily bottlenecked by missing energy and mono-photon searches at NA64 and BaBar. Our global phenomenological analysis reveals that the synergistic theoretical upper bound dictated by these indirect high-energy probes completely eclipses the direct scattering constraints from Borexino. This establishes a strict phenomenological hierarchy: high-intensity cLFV probes and accelerator-based dark sector searches jointly possess the overwhelmingly dominant exclusionary power over direct solar neutrino limits for such microscopic magnetic moment models.

Dark Transition Magnetic Moments of Majorana Neutrinos Mediated by a Dark Photon

Abstract

Standard Model predictions for Majorana neutrino transition magnetic moments (TMMs) are subject to severe chiral and GIM-like suppressions, rendering them vanishingly small. To dynamically generate a macroscopic TMM, we propose a dark sector framework featuring a gauge symmetry, a vector-like lepton doublet, and two complex dark scalars. We demonstrate that while fermion-radiated loop amplitudes identically cancel due to Majorana self-conjugacy, a chirally enhanced dark TMM is successfully generated exclusively through scalar-radiated loops. This mechanism safely shifts the required chirality flip onto the heavy internal fermion line and utilizes a misaligned double-scalar mixing in flavor space to evade the Majorana antisymmetry prohibition. We systematically confront this tensor portal framework with multi-frontier experimental constraints. Since the dark TMM generation is inextricably linked to charged lepton flavor violation, the internal Yukawa couplings are stringently capped by the latest limits from MEG II. Concurrently, the visible-dark kinetic mixing portal is heavily bottlenecked by missing energy and mono-photon searches at NA64 and BaBar. Our global phenomenological analysis reveals that the synergistic theoretical upper bound dictated by these indirect high-energy probes completely eclipses the direct scattering constraints from Borexino. This establishes a strict phenomenological hierarchy: high-intensity cLFV probes and accelerator-based dark sector searches jointly possess the overwhelmingly dominant exclusionary power over direct solar neutrino limits for such microscopic magnetic moment models.

Paper Structure

This paper contains 10 sections, 34 equations, 7 figures, 1 table.

Table of Contents

  1. Introduction
  2. The Dual-Scalar Framework
  3. Dark Sector Lagrangian
  4. Mass Basis and Misalignment
  5. One-Loop Dynamics
  6. Global Constraints
  7. The Visible Sector: $\mu \to e \gamma$
  8. The Dark Sector: Borexino and NA64
  9. Joint Analysis and Cosmology
  10. Conclusions

Figures (7)

  • Figure 1: Feynman Diagram of Neutrino Magnetic Moment.
  • Figure 2: Fermion radiation contribution diagram.
  • Figure 3: Scalar radiation contribution diagram.
  • Figure 4: One-loop Feynman diagram inducing $\mu^- \to e^- \gamma$.
  • Figure 5: Diagram of dark photon-mediated neutrino-electron elastic scattering.
  • ...and 2 more figures