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Unveiling Neutrino Nature with the Diffuse Supernova Background

This paper proposes that the Diffuse Supernova Neutrino Background (DSNB) can determine whether neutrinos are Dirac or Majorana particles by detecting flux alterations caused by resonant chirality flips in magnetorotational supernovae, a signature potentially observable by Hyper-Kamiokande and JUNO within 20 years if a sufficient fraction of core collapses involve strong magnetic fields.

Original authors: Marco Manno, Pablo Martínez-Miravé, Irene Tamborra

Published 2026-08-18

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CC BY 4.0

Original authors: Marco Manno, Pablo Martínez-Miravé, Irene Tamborra

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Neutrinos are ghostly particles that zip through the universe almost without touching anything. They are produced in the nuclear furnaces of stars and in the violent deaths of massive stars, yet they remain one of the most mysterious components of the cosmos. For decades, physicists have debated a fundamental question about their identity: are they their own antiparticles, or are they distinct from them? This distinction, known as whether they are Majorana or Dirac particles, is not just a matter of classification; it holds the key to understanding why the universe is made of matter rather than being an empty void of equal parts matter and antimatter. While scientists have searched for clues in the decay of atoms deep underground, those experiments have not yet provided a definitive answer. Now, a new approach suggests that the answer might be hidden in a faint, cosmic hum of neutrinos that has been traveling across the universe since the first stars began to die.

A team of researchers has proposed a way to solve this puzzle by listening to the "diffuse supernova neutrino background." This background is not a single signal from one star, but a cumulative glow of neutrinos from every massive star that has collapsed and exploded over the history of the universe. While most of these explosions are driven by the intense pressure of neutrinos themselves, a small fraction are thought to be powered by a different mechanism: the rapid spin and immense magnetic fields of the dying star. The researchers focused on these rare, magnetically driven events. They calculated that if neutrinos possess a specific magnetic property, the powerful magnetic fields inside these collapsing stars would act like a switch, flipping the nature of the neutrinos as they escape.

The outcome of this flip depends entirely on whether neutrinos are Majorana or Dirac particles. If they are Majorana, the flip changes a neutrino into an antineutrino, and vice versa, altering the mix of particles that eventually reach Earth. If they are Dirac, the flip turns them into a "sterile" version that cannot be detected by our instruments at all. This means that the total number of neutrinos we detect from this cosmic background, and their energy levels, would look different depending on which type of particle they are. The researchers simulated these scenarios using data from two massive underground detectors, Hyper-Kamiokande in Japan and JUNO in China, which are designed to catch these elusive particles.

The study finds that if the universe contains a sufficient number of these magnetically driven supernovae—specifically, if they make up more than about 12 percent of all stellar collapses—these two future detectors could distinguish between the two types of neutrinos with high confidence after twenty years of observation. The signal would be most visible in the high-energy neutrinos, where the difference between the two possibilities becomes stark. If the fraction of these special supernovae is even higher, reaching about 20 percent, the detectors could confirm the answer with even greater certainty. This result holds true regardless of the specific arrangement of neutrino masses, which is another major unknown in the field.

This work does not claim to have solved the mystery yet, but it maps out a clear path forward. It suggests that the diffuse background of neutrinos, long considered just a theoretical prediction, is on the verge of becoming a practical tool for discovery. By combining data from two of the world's most sensitive neutrino observatories, scientists may soon be able to determine the true nature of the neutrino. This discovery would not only answer a decades-old question about the identity of these ghost particles but would also provide a crucial piece of the puzzle regarding how the universe came to be filled with the matter that makes up our world. The key lies in waiting for the right fraction of magnetically charged stellar deaths to leave their mark on the cosmic neutrino background.

Technical Summary: Unveiling Neutrino Nature with the Diffuse Supernova Background

Problem Statement
The fundamental nature of neutrinos—whether they are Dirac or Majorana particles—remains one of the most significant unresolved questions in modern physics. This distinction is inextricably linked to the origin of neutrino mass and the conservation of total lepton number. While the primary experimental strategy to address this involves the search for neutrinoless double beta decay ( $0\nu\beta\beta$ ), no evidence has yet been found. Alternative probes, such as the cosmic neutrino background (C $\nu$ B), face extreme experimental challenges and uncertainties regarding local relic neutrino clustering. This paper investigates whether the Diffuse Supernova Neutrino Background (DSNB) can serve as a viable diagnostic tool to distinguish between Dirac and Majorana neutrinos.

Methodology
The authors model the DSNB flux as a superposition of contributions from standard neutrino-driven core-collapse supernovae ( $\nu$ -SN) and a subset of magnetorotational (MR) collapses. The total flux is expressed as $\Phi_{\text{DSNB}} = (1 - f_{\text{MR}})\Phi_{\nu\text{-SN}} + f_{\text{MR}}\Phi_{\text{MR}}$ , where $f_{\text{MR}}$ represents the fraction of magnetorotational events.

The core of the methodology relies on flavor conversion physics within the collapsing stellar cores:

Flavor Conversion Mechanisms: The study accounts for standard Mikheyev–Smirnov–Wolfenstein (MSW) resonances, which affect Dirac and Majorana neutrinos identically. Crucially, it incorporates magnetically-driven resonant conversions (B-res) occurring in MR collapses, provided neutrinos possess magnetic dipole moments $\mu \gtrsim 10^{-14} \mu_B$ .
Dirac vs. Majorana Distinction:
- Majorana Neutrinos: Magnetic interactions allow resonant chirality flips that convert active neutrinos into active antineutrinos (and vice versa), effectively reshuffling flavors.
- Dirac Neutrinos: Magnetic interactions convert active neutrinos/antineutrinos into opposite-chirality states. These opposite-chirality states do not participate in Standard Model weak interactions and are therefore undetectable, leading to a depletion of the observable flux.
Simulation and Parameters: The authors utilize hydrodynamical simulations for neutrino-driven explosions (representing masses $<15 M_\odot$ and $>15 M_\odot$ ) and a specific 3D magnetorotational model ( $13 M_\odot$ ) to compute time-integrated neutrino spectra. They assume a constant MR fraction $f_{\text{MR}}$ over redshift and marginalize over the fraction of black-hole-forming collapses ( $f_{\text{BH}}$ ).
Detector Modeling: The expected event rates are calculated for two upcoming detectors: Hyper-Kamiokande (loaded with gadolinium) and JUNO, assuming a 20-year combined exposure. The analysis focuses on inverse beta decay ( $\bar{\nu}_e + p \to e^+ + n$ ) events, incorporating detector energy resolution, efficiency, and background models (primarily atmospheric neutrinos).
Statistical Analysis: A frequentist approach is employed using a test statistic $\Delta\chi^2$ to compare the "true" hypothesis (Dirac or Majorana with magnetic conversion) against the "test" hypothesis. Systematic uncertainties, including background normalization and supernova rate uncertainties, are treated as nuisance parameters.

Key Contributions

Theoretical Framework: The paper derives the specific resonance conditions for magnetically-driven flavor conversion in Dirac neutrinos, a scenario previously less explored compared to the Majorana case. It demonstrates that for Dirac particles, these resonances occur at larger radii ( $10^4$ – $10^5$ km) compared to Majorana particles ( $10^3$ km) and result in the production of sterile opposite-chirality states.
Flux Signatures: The authors show that magnetically-driven conversion imprints distinct signatures on the DSNB $\bar{\nu}_e$ $\overset{ν}{ˉ}_{e}$ energy spectrum, particularly in the high-energy tail ( $E_\nu \gtrsim 15$ $E_{ν} ≳ 15$ MeV).
- In the Normal Ordering (NO) scenario, Majorana neutrinos exhibit an excess relative to the standard MSW-only prediction, while Dirac neutrinos show a deficit.
- In the Inverted Ordering (IO) scenario, both cases show depletion relative to MSW-only, but the magnitude of depletion differs significantly between Dirac and Majorana cases.
Feasibility Study: The work quantifies the statistical power of the combined Hyper-Kamiokande-Gd and JUNO experiments to resolve these differences.

Results
The analysis projects the sensitivity of a 20-year combined dataset from Hyper-Kamiokande-Gd and JUNO:

Discrimination Capability: The experiments can distinguish between Dirac and Majorana neutrinos at 90% confidence level (CL) if the fraction of magnetorotational collapses ( $f_{\text{MR}}$ ) exceeds approximately 12% (for Normal Ordering) or 9% (for Inverted Ordering).
Higher Confidence: A 99% CL discrimination is achievable if $f_{\text{MR}}$ exceeds 19–21% (Normal Ordering) or 13–14% (Inverted Ordering).
Robustness: These results hold independently of the neutrino mass ordering. The sensitivity is most robust when the neutrino magnetic moment is $\gtrsim 10^{-14} \mu_B$ , ensuring adiabatic conversion.
Limitations: The discrimination power is most sensitive to the high-energy tail of the spectrum ( $>18$ MeV), where atmospheric neutrino backgrounds dominate. Consequently, the ability to distinguish neutrino nature relies heavily on reducing background uncertainties and the successful implementation of gadolinium loading in Hyper-Kamiokande for neutron tagging.

Significance
The paper establishes the DSNB as a critical, untapped gateway to determining the fundamental identity of the neutrino. It argues that if a sufficient fraction of core-collapse supernovae are magnetorotational and neutrinos possess magnetic moments above the threshold of $10^{-14} \mu_B$ , the upcoming generation of neutrino telescopes can resolve the Dirac vs. Majorana question without relying on neutrinoless double beta decay or the cosmic neutrino background. This finding positions the DSNB not merely as a probe of stellar populations, but as a high-energy frontier for fundamental particle physics. The authors emphasize that achieving this sensitivity requires precise constraints on the cosmic core-collapse rate and further mitigation of experimental backgrounds, objectives that are already central to future DSNB measurement plans.

Technical Summary: Unveiling Neutrino Nature with the Diffuse Supernova Background

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