[Paper Review] A chiral gauge-invariant model for Majorana neutrinos

I. Alikhanov, E. A. Paschos|arXiv (Cornell University)|Feb 26, 2019

Neutrino Physics Research22 references4 citations

TL;DR

This paper proposes a chiral gauge-invariant U(1)R model embedding right-handed Majorana neutrinos that couple to a new neutral gauge boson X via purely axial-vector interactions. The model ensures ultraviolet finiteness through anomaly cancellation and predicts new signatures in (anti)neutrino–electron scattering and coherent neutrino–nucleus scattering, with coupling bounds derived from the ρ parameter and experimental data, showing up to 20% unknown contributions possible in current measurements.

ABSTRACT

The article investigates the possibility that right-handed neutrinos are Majorana particles embedded in an abelian multiplicative $U(1)_R$ factor, with the Lagrangian in the new factor being invariant under chiral gauge transformations. Majorana neutrinos couple to charged and neutral currents, producing new signatures to (anti)neutrino-electron elastic scattering and an additional term to coherent scattering of neutrinos on atomic nuclei. The model is ultraviolet complete and depends on one extra hypercharge correlating the reactions and providing upper bounds for the new coupling constant.

Motivation & Objective

• To construct a gauge-invariant model where right-handed Majorana neutrinos couple to a new U(1)R gauge boson X through chiral (axial-vector) interactions.
• To ensure ultraviolet finiteness by canceling triangle anomalies and preserving chiral gauge symmetry.
• To derive phenomenological constraints on the new coupling constant and X boson mass from the ρ parameter and neutrino scattering experiments.
• To explore detectable signatures in (anti)neutrino–electron elastic scattering and coherent neutrino–nucleus scattering.
• To correlate different neutrino-induced reactions through shared U(1)R hypercharges, enabling cross-experiment consistency checks.

Proposed method

• Introduce a new abelian U(1)R gauge symmetry with chiral invariance under γ⁵ transformations, ensuring gauge invariance of the Lagrangian for right-handed Majorana neutrinos.
• Implement a Higgs mechanism using a standard Higgs doublet φ and a new scalar singlet σ to generate masses for fermions and the X gauge boson.
• Enforce anomaly cancellation via a specific assignment of U(1)R hypercharges, reducing the number of independent charges to one for quarks and leptons.
• Derive the effective couplings of the X boson to neutrinos and electrons, including mixing with the Z boson, and compute the resulting modifications to cross sections.
• Use the ρ parameter and Z boson invisible decay branching ratios to constrain the U(1)R coupling constant and X boson mass.
• Analyze differential cross sections for (anti)neutrino–electron scattering and coherent neutrino–nucleus scattering, accounting for new axial-vector contributions.

Experimental results

Research questions

• RQ1 Can Majorana neutrinos couple to a neutral gauge boson X in a chiral gauge-invariant framework?
• RQ2 What are the phenomenological consequences of such a coupling for neutrino–electron scattering and coherent scattering on nuclei?
• RQ3 How do the constraints from the ρ parameter and Z invisible decays limit the coupling strength and mass of the new X boson?
• RQ4 To what extent can new contributions to neutrino–electron scattering be hidden within current experimental uncertainties?
• RQ5 Can beam-dump or spallation sources produce detectable signals from Majorana neutrinos via the X boson exchange?

Key findings

• The model is ultraviolet complete, with triangle anomalies canceled through a consistent assignment of U(1)R hypercharges, reducing the number of independent charges to one.
• The new X boson couples exclusively to right-handed Majorana neutrinos via a purely axial-vector current, preserving chiral gauge invariance.
• Constraints from the ρ parameter and Z invisible decays are not the most restrictive; instead, (anti)neutrino–electron scattering data imply that unknown contributions up to 20% are still possible.
• The differential cross section for (anti)neutrino–electron scattering is modified by additional axial-vector couplings, with the effect becoming significant at low momentum transfer.
• Coherent neutrino–nucleus scattering receives a baryonic current enhancement due to the axial-vector nature of the interaction, increasing the cross section sensitivity.
• The coupling constant of the X boson is bounded by experimental data, with the TEXONO experiment's limit closely matching the value required for relic dark matter abundance in the early universe.