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

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$\nu$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.