Exploring the Neutrino Mass Hierarchy Probability with Meteoritic Supernova Material, {nu}-Process Nucleosynthesis, and {theta}13 Mixing

There is recent evidence that some SiC X grains from the Murchison meteorite may contain supernova-produced {\nu}-process 11B and or 7Li encapsulated in the grains. The synthesis of 11B and 7Li via neutrino-induced nucleon emission (the {\nu} -process) in supernovae is sensitive to the neutrino mass hierarchy for finite sin^2(2{\theta}13) > 0.001}. This sensitivity arises because, when there is 13 mixing, the average electron neutrino energy for charged-current neutrino reactions is larger for a normal mass hierarchy than for an inverted hierarchy. Recent constraints on {\theta}13 from the Daya Bay, Double Chooz, MINOS, RENO and T2K collaborations all suggest that indeed sin^2(2{\theta}13) > 0.001}. We examine the possible implications of these new results based upon a Bayesian analysis of the uncertainties in the measured meteoritic material and the associated supernova nucleosynthesis models. We show that although the uncertainties are large, they hint at a marginal preference for an inverted neutrino mass hierarchy. We discuss the possibility that an analysis of more X grains enriched in Li and B along with a better understanding of the relevant stellar nuclear and neutrino reactions could eventually reveal the neutrino mass hierarchy.

💡 Research Summary

The paper investigates whether the neutrino mass hierarchy (normal versus inverted) can be constrained using the ν‑process nucleosynthesis of light isotopes ⁷Li and ¹¹B observed in presolar SiC X grains from the Murchison meteorite. The authors begin by reviewing the three‑flavor neutrino mixing framework, emphasizing that recent reactor and accelerator experiments have established a relatively large θ₁₃ (sin²2θ₁₃ > 0.001). In core‑collapse supernovae, the ν‑process produces ⁷Li and ¹¹B through neutrino‑induced nucleon emission in the He‑rich outer layers. Because the average energy of electron neutrinos (νₑ) and antineutrinos ( ν̄ₑ) is altered by matter‑enhanced oscillations, the yields of these isotopes become sensitive to the mass hierarchy when θ₁₃ is sizable. In the normal hierarchy, an adiabatic 13‑mixing resonance boosts νₑ energies, enhancing charged‑current reactions that increase ⁷Be (which decays to ⁷Li) and ¹¹C (which decays to ¹¹B). In the inverted hierarchy, the resonance affects  ν̄ₑ, leading to a modest increase of ⁷Li and ¹¹B, but the effect is smaller because  ν̄ₑ already have higher average energies. Earlier studies, based on a single 16.2 M⊙ supernova model, showed that for sin²2θ₁₃ > 10⁻³ the ratio ⁷Li/¹¹B differs appreciably between the two hierarchies.

The meteoritic evidence comes from a recent analysis of 1 kg of Murchison material, which identified 12 SiC X grains (presolar supernova condensates). Seven of these grains display measurable Li and B isotopic anomalies: ⁷Li/⁶Li = 11.83 ± 0.29 (solar value 12.06) and ¹¹B/¹⁰B = 4.68 ± 0.31 (solar value 4.03). Assuming the observed ratios are a sum of a solar component plus a ν‑process contribution, the authors derive a ν‑process ⁷Li/¹¹B ratio of –0.31 ± 0.42, which translates into a 2σ upper limit of 0.53 (95 % confidence). They also use the measured elemental abundances (Li/Si and B/Si) to correct for possible non‑ν‑process Li/B contributions, arriving at a consistent upper bound.

To assess the hierarchy preference, the authors employ a Bayesian framework. They define likelihood functions P(D|M_i) that incorporate uncertainties in supernova progenitor mass (10–25 M⊙), key nuclear reaction rates (e.g., 3α→¹²C, ¹²C(α,γ)¹⁶O), and neutrino spectral parameters. Priors for the two hierarchy models are taken as equal (50 % each). By integrating over the multidimensional parameter space, they obtain posterior probabilities P(normal|D) ≈ 0.26 and P(inverted|D) ≈ 0.74. Thus, despite large uncertainties and the fact that the data provide only an upper limit, the analysis yields a marginal preference for the inverted hierarchy.

The paper concludes that the current meteoritic data are insufficient for a definitive determination, but they demonstrate that ν‑process nucleosynthesis combined with presolar grain analysis can, in principle, probe the neutrino mass ordering. Future progress requires (i) a larger sample of X grains with precise Li and B isotopic measurements, (ii) improved experimental constraints on the relevant nuclear reaction rates, and (iii) more sophisticated supernova neutrino transport models that reduce the theoretical spread in predicted ⁷Li and ¹¹B yields. With these advances, the ν‑process could become a complementary astrophysical tool alongside terrestrial oscillation experiments for resolving the neutrino mass hierarchy.