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Constraining neutrino charges at beam experiments

Jack D. Shergold, Martin Spinrath

TL;DR

This work proposes a novel, model-independent method to bound the neutrino electric charge by detecting the azimuthal magnetic field generated by dense, ultrarelativistic neutrino bunches with sensitive magnetometers along the beam. The analysis derives the neutrino-induced field in both idealized and realistic (Gaussian) bunch scenarios, showing how the signal scales with bunch parameters and time resolution, and quantifies potential bounds for existing and planned beam facilities. The study also assesses extensions to neutrino electromagnetic moments and to new long-range forces, finding that while dipole-moment constraints are not competitive, the method can probe light mediators and dark-photon scenarios under favorable conditions. Overall, the approach could push model-independent bounds on $|q_\nu|$ to the $\sim 10^{-13}$–$10^{-14}$ level with upgrades (e.g., LBNF-u) and current beams (e.g., J-PARC), making it a promising avenue for exploring beyond-Standard-Model electromagnetic couplings of neutrinos.

Abstract

We propose a new method to constrain neutrino charges at neutrino beam experiments. Uncharged in the Standard Model, evidence for a neutrino electric charge would be a smoking gun for new physics, shedding light on the Dirac or Majorana nature of neutrinos, and giving insight into the origin of charge quantization. We find that using the most sensitive magnetometers available, existing beam experiments could constrain neutrino charges $|q_ν| \lesssim 10^{-13}$, in units of the electron charge, while future upgrades could strengthen these bounds significantly. We also discuss electromagnetic dipole moments and show that our proposal is highly sensitive to new long-range forces.

Constraining neutrino charges at beam experiments

TL;DR

This work proposes a novel, model-independent method to bound the neutrino electric charge by detecting the azimuthal magnetic field generated by dense, ultrarelativistic neutrino bunches with sensitive magnetometers along the beam. The analysis derives the neutrino-induced field in both idealized and realistic (Gaussian) bunch scenarios, showing how the signal scales with bunch parameters and time resolution, and quantifies potential bounds for existing and planned beam facilities. The study also assesses extensions to neutrino electromagnetic moments and to new long-range forces, finding that while dipole-moment constraints are not competitive, the method can probe light mediators and dark-photon scenarios under favorable conditions. Overall, the approach could push model-independent bounds on to the – level with upgrades (e.g., LBNF-u) and current beams (e.g., J-PARC), making it a promising avenue for exploring beyond-Standard-Model electromagnetic couplings of neutrinos.

Abstract

We propose a new method to constrain neutrino charges at neutrino beam experiments. Uncharged in the Standard Model, evidence for a neutrino electric charge would be a smoking gun for new physics, shedding light on the Dirac or Majorana nature of neutrinos, and giving insight into the origin of charge quantization. We find that using the most sensitive magnetometers available, existing beam experiments could constrain neutrino charges , in units of the electron charge, while future upgrades could strengthen these bounds significantly. We also discuss electromagnetic dipole moments and show that our proposal is highly sensitive to new long-range forces.

Paper Structure

This paper contains 8 sections, 21 equations, 1 figure, 1 table.

Table of Contents

  1. Introduction
  2. Neutrino magnetic fields
  3. Realistic example
  4. Additional prospects
  5. Other electromagnetic moments
  6. Additional long range forces
  7. Conclusions
  8. Imperfect time resolution and polychromatic beams

Figures (1)

  • Figure 1: Sensitivity to the neutrino charge for left panel: the most promising existing and upcoming neutrino experiments, cf. Table \ref{['tab:expParams']}, and right panel: a toy experiment with variable bunch size, but otherwise the same beam parameters as J-PARC. In both panels, the solid lines assume a magnetometer with sensitivity comparable to existing SQUIDs, $B_\mathrm{ref} \simeq 10^{-15}\,\mathrm{T}$, while the dot-dashed lines assume sensitivity to $B_\mathrm{ref}\simeq 10^{-17}\,\mathrm{T}$, akin to a SERF magnetometer. The horizontal dashed line represents the current, most stringent, model-independent laboratory bounds on the individual flavor-diagonal charges, $|q_{\nu_\alpha}| \lesssim 10^{-13}$AtzoriCorona:2022jebA:2022acyGiunti:2023yha.