Solar Flares as a Probe of Neutrino Nature: Distinguishing Dirac and Majorana via Resonant Spin-Flavor Precession

TL;DR

The paper investigates whether Resonant Spin-Flavor Precession (RSFP) in solar magnetic fields can reveal the Dirac vs Majorana nature of neutrinos. Using a density-matrix/open-quantum-system approach, it models RSFP in the Sun with three magnetic-field profiles and includes collisional decoherence, showing standard MeV solar neutrinos have resonances in the dense core and outer fields are largely inaccessible, while GeV solar-flare neutrinos experience resonant conversion in the tachocline and convective zones where strong fields exist. This leads to measurable differences in scattering cross sections: in neutrino-electron scattering, asymmetries up to about 16.9% in certain tachocline scenarios; in CE$\nu$NS, Dirac flux loss can be ~36% while Majorana flux remains largely intact, offering a robust discriminator or improved μν limits if no signal is observed. The results motivate a multi-messenger observational strategy combining real-time gamma-ray triggers from observatories like HAWC with high-statistics detectors (Hyper-K, IceCube-Gen2) to test neutrino nature and electromagnetic properties with solar-flare neutrinos.

Abstract

Resonant Spin-Flavor Precession (RSFP) of solar neutrinos is studied using the quantum density matrix formalism, explicitly taking into account collisional decoherence and solar matter density profiles. The transition probabilities for standard $^8$B solar neutrinos ($E \approx 10$ MeV) and ultra-high-energy flare neutrinos ($E \gtrsim 1$ GeV) under three magnetic field hypotheses: core-concentrated (Wood-Saxon), tachocline-confined (Gaussian), and turbulent convective (Power Law) are compared. For standard LMA parameters, we show the resonance for 10 MeV neutrinos is strictly confined to the deep solar core ($r < 0.2 R_\odot$), rendering standard solar neutrinos insensitive to outer magnetic fields. Conversely, for 1 GeV flare neutrinos, the resonance shifts to the tachocline and convective zones, where strong fields ($B \sim 50$ kG) drive efficient spin conversion. We apply this effect to compute the difference between Dirac or Majorana neutrino scattering cross section as electron-neutrino scattering and Coherent Elastic Neutrino-Nucleus Scattering (CE$ν$NS). We show that significant asymmetry in these cross section are possible allowing in case of detection to distinguish between Dirac or Majorana neutrinos. In case of null observation, we show that this method can potentially improved the limit on the neutrino magnetic moment by one order to magnitude compared to current limits.

Figures (4)

• Figure 1: Contour plot of the neutrino helicity parameter $S_{\parallel}$ in the ($B$, $\mu_\nu$) parameter space, calculated for 1.15 GeV flare neutrinos resonating in the solar tachocline. The orange dashed line marks the threshold for a detectable signal, $S_{\parallel} > -0.9$. The red dashed line indicates the current upper limit on the neutrino magnetic moment from Borexino, $\mu_\nu < 2.8 \times 10^{-11} \mu_B$. The cyan dotted line represents a typical magnetic moment value predicted by some Standard Model extensions, $\mu_\nu = 10^{-12} \mu_B$. The vertical white dotted line marks the standard estimate for the peak tachocline magnetic field, $B \approx 50$ kG.
• Figure 2: Neutrino resonance energy ($E_{res}$) as a function of solar radius, calculated for standard Large Mixing Angle (LMA) oscillation parameters ($\Delta m^2 \approx 7.5 \times 10^{-5}$ eV$^2$). The plot illustrates the neutrino energy needed to fullfill the Resonant Spin-Flavor Precession (RSFP) condition across different solar regions. The horizontal green dotted line marks the energy of standard $^8$B solar neutrinos ($E \approx 10$ MeV), showing that their resonance is confined to the high-density radiative core ($r < 0.2 R_\odot$), where the adiabatic transition efficiency is suppressed. The horizontal purple dotted line indicates the energy of solar flare neutrinos ($E \sim 1$ GeV). The vertical red dashed line and orange shaded region highlight the tachocline ($r \approx 0.71 R_\odot$), the interface between the radiative and convective zones, where the resonance for 1 GeV neutrinos coincides with strong toroidal magnetic fields, facilitating efficient spin-flavor conversion.
• Figure 3: Evolution of the parallel spin component $S_\parallel$ for a 1 GeV solar flare neutrino traversing the solar tachocline. The simulation assumes a Gaussian magnetic field profile centered at $r = 0.71 R_\odot$ with a peak strength of $B = 50$ kG and a neutrino magnetic moment at the Borexino limit ($\mu_\nu = 2.8 \times 10^{-11} \mu_B$). The shaded blue area indicates the resonance region where the adiabaticity condition allows for a significant conversion from the initial left-handed state ($S_\parallel = -1$) to a mixed state, demonstrating the high efficiency of RSFP at this energy scale
• Figure 4: Helicity evolution ($S_\parallel$) for a 1 GeV neutrino propagating through the solar convective zone, using an adjusted density profile to explicitly capture the resonance crossing at $r \approx 0.72 R_\odot$ (marked by the vertical orange dotted line). The green curve shows the macroscopic deviation from the pure left-handed state ($S_\parallel = -1$) initiated by the resonance. The final stabilized value of $S_\parallel \approx -0.88$ illustrates that measurable depolarization effects are theoretically expected in the outer solar layers for high-energy neutrinos.