Supernova Neutrinos May Reveal If Particles Are Their Own Antimatter

Scientists are increasingly utilising supernova bursts to investigate fundamental neutrino properties, and a new study by Delepine and Yebra explores this potential through the phenomenon of Resonant Spin-Flavor Precession (RSFP). Their research, conducted without institutional affiliation details provided, demonstrates how the cooling duration of supernova SN1987A places significant constraints on standard RSFP models for Dirac neutrinos. Importantly, the authors show that RSFP-induced helicity changes create distinct, detectable signatures in Coherent Elastic Neutrino-Nucleus Scattering (CEνNS) experiments, potentially differentiating between Dirac neutrinos and the more elusive Majorana fermions. This work proposes a novel ratio-based experimental strategy, leveraging high-energy neutrino tails, to minimise astrophysical uncertainties and promises to probe neutrino magnetic moments down to levels two orders of magnitude beyond current solar constraints, representing a substantial step towards understanding the fundamental nature of these particles.

Supernova Neutrino Oscillations and the Distinction Between Dirac and Majorana Fermions

Scientists have uncovered a mechanism by which core-collapse supernova neutrinos can undergo resonant spin-flavor precession (RSFP), potentially revealing their fundamental nature as either Dirac or Majorana particles. This work addresses a long-standing conflict between standard RSFP models for Dirac neutrinos and the observed cooling duration of supernova SN1987A.
Researchers demonstrate that, for neutrino magnetic moments ranging from 10−14 to 10−12μB, adiabatic conversion within the outer stellar envelope (R 1000km) can induce macroscopic helicity inversion without violating established cooling bounds. Conversely, for Majorana neutrinos, the total flux remains unchanged, but a modification to the spectral decomposition of the cross section arises from the transition of left-handed electron neutrinos to right-handed muon or tau anti-neutrinos.

The study proposes an innovative experimental strategy leveraging the high-energy neutrino tail (E ∼1 GeV) to normalise signals and minimise astrophysical uncertainties. By employing a ratio-based approach, researchers aim to effectively cancel out astrophysical ambiguities, enabling future detectors to differentiate between Dirac and Majorana neutrinos and probe magnetic moments down to 10−14μB.

This represents a two-orders-of-magnitude improvement over current solar limits. The research utilises the quantum density matrix formalism to model neutrino propagation within the dense supernova environment, accounting for non-linear self-interaction terms crucial for accurate predictions. The investigation identifies a resonant conversion window occurring in the outer layers of the progenitor star at approximately 104km, where neutrinos have thermally decoupled but sufficient magnetic field and matter density remain to facilitate RSFP.

This process leads to conversion into sterile states detectable via CEνNS, all while preserving the observed supernova cooling timescale. Furthermore, the high-energy neutrino tail, unaffected by RSFP, serves as a crucial calibration tool, allowing for the disentanglement of magnetic effects from inherent astrophysical uncertainties.

The adiabaticity parameter, γSN, is naturally large due to the intense supernova magnetic fields, ensuring efficient flavour conversion even for minimal magnetic moments. The temporal structure of the neutrino burst from SN1987A, lasting approximately 10, 15 seconds, provides a critical constraint on RSFP models.

Standard diffusion timescales estimate neutrino escape at around 10 seconds, but rapid escape due to sterile neutrino production would shorten this to approximately 1 second for Dirac neutrinos. This imposes a limit of μν ≲10−12μB. Researchers addressed the limitations of standard RSFP models for Dirac neutrinos imposed by the cooling duration of SN1987A by focusing on the outer stellar envelope where resonant conversion could occur after thermal decoupling.

Calculations demonstrate that adiabatic conversion within the envelope, at radii of approximately 104km, induces macroscopic helicity inversion for neutrino magnetic moments ranging between 10−14μB and 10−22μB without violating established cooling bounds. The work employed the Liouville-von Neumann equation to govern the evolution of the neutrino ensemble’s density matrix, incorporating a dissipator to account for collisional decoherence.

The supernova Hamiltonian, central to the calculations, included vacuum, matter, magnetic, and neutrino-neutrino interaction terms. Matter potential was calculated using electron and neutron number densities, recognising that neutron dominance in the core fundamentally alters resonance conditions compared to solar scenarios.

A transverse magnetic field, ranging from 1012 G to 1015 G, was incorporated into the Hamiltonian to model magnetic interactions, representing a field strength approximately 1010times greater than that found in the solar tachocline. Furthermore, the study accounted for neutrino self-interactions arising from coherent forward scattering, coupling the evolution of neutrino modes through an integral over all other modes.

To assess the efficiency of RSFP conversion, the adiabaticity parameter γSN was computed, revealing that even small magnetic moments, around 10−20μB, can facilitate efficient transitions due to the intense supernova magnetic fields. This research demonstrates that neutrino magnetic moments within the range of 10−14 to 10−12 J/T induce this conversion.

The study focuses on SN1987A, where the cooling duration places severe constraints on standard RSFP models for Dirac neutrinos, and reveals how RSFP can alter neutrino helicity. For Dirac neutrinos, the work predicts a substantial flux deficit due to sterile conversion, indicating a measurable reduction in detected neutrinos.

Conversely, for sterile neutrinos, the total flux remains unchanged, but a modification to the spectral decomposition of the cross section is expected, resulting from the transition of left-handed electron neutrinos to right-handed or anti-neutrinos. This distinction provides a pathway to differentiate between Dirac and sterile neutrino types through observation of coherent elastic neutrino-nucleus scattering.

An experimental strategy is proposed utilizing the high-energy neutrino tail above 10 GeV, which largely evades RSFP, to normalize the signal and minimize astrophysical uncertainties. This ratio-based approach effectively cancels uncertainties, enabling future detectors to probe magnetic moments down to 10−14 J/T, representing a two-orders-of-magnitude improvement over current solar limits. The research highlights the potential for distinguishing the fundamental nature of the neutrino and refining our understanding of supernova dynamics through precise measurements of neutrino interactions.

Resonant Neutrino Conversion and Distinguishing Dirac from Majorana Particles in Supernova Environments

Scientists have demonstrated that the outer envelope of a core-collapse supernova provides a suitable environment for resonant spin-flavor precession (RSFP), a process that efficiently converts neutrino helicity while remaining consistent with observational constraints from supernova 1987A. This analysis identifies a region beyond 1000km from the star’s core where the resonance occurs naturally for neutrinos after they have thermally decoupled, enabling complete adiabatic conversion for neutrino magnetic moments between 10−14 and 10−12 μB.

The resulting signatures in coherent elastic neutrino-nucleus scattering (CEνNS) detectors differ depending on neutrino type; Dirac neutrinos would exhibit a substantial flux deficit due to conversion into sterile states, while Majorana neutrinos would show a hardening of the spectral recoil. A novel experimental strategy utilising the high-energy neutrino tail at approximately 1 GeV has been proposed to normalise signals and mitigate astrophysical uncertainties.

This ratio-based approach focuses on the relative measurement between thermal and high-energy events, effectively cancelling out uncertainties related to distance and luminosity. This work establishes a clear path toward distinguishing between Dirac and Majorana neutrinos through supernova observation.

Furthermore, the proposed methodology could improve current limits on neutrino magnetic moments by over two orders of magnitude, reaching a sensitivity of 10−14 μB. The authors acknowledge that the analysis relies on assumptions about the outer stellar envelope and the adiabaticity of the conversion process, which could introduce some limitations. Future research will likely focus on refining models of the supernova environment and exploring the potential for multi-messenger observations to further constrain neutrino properties.