Are neutrinos Majorana? Fixed-target and high-energy astrophysical searches decide
Gabriela Barenboim, Mauricio Bustamante, Qinrui Liu
Abstract
Determining whether the neutrino is a Dirac or Majorana fermion remains a fundamental open question. Conventional searches rely on neutrinoless double beta decay, but this electron-only channel suffers from blind spots. We propose a new, complementary probe to overcome this limitation. A heavy neutral lepton (HNL) triggers a high-energy shift in how the active neutrino flavors (, , ) mix -- but only if the neutrinos are Majorana. For GeV-scale HNLs, the upcoming beam-dump experiment SHiP can discover the HNL and measure how it mixes with the active flavors. Separately, the scattering of TeV--PeV astrophysical neutrinos can resolve the HNL, revealing a shift in the proportions of each flavor arriving at Earth that could be detected by neutrino telescopes, regardless of the unknown flavor composition at the astrophysical neutrino sources. Because this flavor shift is most sensitive to the muon and tau sectors, it bypasses the blind spots of neutrinoless double beta decay. A correlated signal at SHiP and next-generation neutrino telescopes would prove that neutrinos are Majorana; its absence would point to them being Dirac.
AI Impact Assessments
(1 models)Scientific Impact Assessment
Core Contribution
This paper proposes a fundamentally new strategy for determining the Majorana vs. Dirac nature of neutrinos by correlating two independent experimental programs: the SHiP beam-dump experiment (measuring HNL mass and couplings) and TeV–PeV astrophysical neutrino flavor composition measurements at neutrino telescopes (IceCube, KM3NeT, Baikal-GVD, and future facilities). The central insight is that a Majorana HNL induces a momentum-dependent threshold correction to the active neutrino mass matrix at Q ∼ M, which modifies the effective mixing parameters at high energies. This correction vanishes identically for Dirac HNLs, providing a clean discriminator. The approach bypasses the well-known blind spots of neutrinoless double beta decay (0νββ), which probes only the electron sector and can be suppressed by Majorana phase cancellations.
Methodological Rigor
The theoretical framework is carefully constructed. The authors derive the one-loop threshold matching correction in the Low Energy Effective Field Theory below the electroweak scale, providing both the full expression (Eq. 5) and physically transparent approximations for light (M ≪ M_W) and heavy (M ≫ M_h) HNL regimes. The perturbative expansion of mixing angle corrections (Eqs. 15–20) provides analytical insight, while full Autonne-Takagi numerical diagonalization handles the non-perturbative regime including level crossings.
The statistical framework employs a frequentist profile-likelihood approach with appropriate treatment of nuisance parameters. Critically, the unknown astrophysical source composition f_{e,S} is floated freely in [0,1], ensuring results are robust against astrophysical ignorance. The application of Chernoff's theorem for boundary-constrained parameters and appropriate trials factors demonstrates statistical sophistication. However, the Asimov dataset approach, while standard for projections, does not capture the impact of statistical fluctuations.
A notable strength is the careful distinction between the physical, scheme-independent threshold correction and the scheme-dependent RG running kinks that appear in MS-bar calculations—an important clarification that strengthens the theoretical foundation.
Potential Impact
Particle physics: If realized, this program would provide model-independent evidence for the Majorana nature of neutrinos without directly observing lepton number violation—a conceptually novel approach. The projected upper limits on |U_{μN}|² and |U_{τN}|² from astrophysical neutrinos would become world-leading above a few GeV, surpassing collider bounds by orders of magnitude.
Astroparticle physics: The paper establishes astrophysical neutrino flavor composition as a precision tool for fundamental physics beyond its traditional role. The detailed multi-detector projections (2040, 2050) provide concrete targets for the neutrino telescope community.
Experimental planning: The three-experiment logic (LHC null + SHiP discovery + astrophysical flavor shift) provides a clear roadmap that could influence experimental priorities and design choices for next-generation facilities.
Timeliness & Relevance
The timing is excellent. SHiP is approaching construction, IceCube has just released updated flavor measurements, and the community is planning next-generation neutrino telescopes (IceCube-Gen2, HUNT, P-ONE). The Majorana/Dirac question remains among the highest-priority open questions in particle physics. The paper addresses a genuine gap: complementary probes of the neutrino nature beyond 0νββ are scarce and urgently needed, particularly given the normal-ordering blind spot.
Strengths
1. Novel observable: Using high-energy astrophysical neutrino flavor as a Majorana discriminator is genuinely original and opens a new phenomenological direction.
2. Comprehensive treatment: The 47-page paper with extensive appendices thoroughly explores the parameter space, including mass ordering effects, Majorana phase dependence, and multiple HNL coupling scenarios.
3. Robustness analysis: The treatment of the unknown source composition as a free parameter, demonstration of resilience to Majorana phases for μ/τ-philic HNLs, and the geometric analysis of why certain configurations evade detection are particularly strong.
4. Complementarity mapping: The R-ratio (Eq. 36) quantifying the relative discovery power of astrophysical vs. 0νββ searches provides a useful framework for the community.
5. Concrete benchmarks: Seven benchmark points with explicit discovery significances provide testable predictions.
Limitations
1. Experimental feasibility gap: The required flavor measurement precision (5–15% shifts) demands multi-detector networks not yet built, pushing firm conclusions to 2040–2050. The 2040 projections using only existing km³-scale telescopes show limited discovery potential.
2. Source composition dependence: Discovery significance is strongly dependent on the astrophysical production mechanism. Full pion decay benchmarks (A, D, E) yield null significance, while only muon-damped scenarios (B, C, F, G) achieve 5σ. This is a significant caveat since the dominant production mechanism remains uncertain.
3. Single HNL assumption: The minimal one-HNL extension may be oversimplified; realistic seesaw models typically require ≥2 HNLs for viable leptogenesis, which would complicate the flavor fingerprint interpretation.
4. Parameter degeneracy: The M³|U_{αN}|² degeneracy in the astrophysical measurement means it cannot independently constrain HNL parameters without SHiP input, limiting standalone utility.
5. No event-level simulation: The flavor composition analysis relies on scaled projections rather than full detector simulations, and the SHiP complementarity contours use approximate semi-analytic extrapolations.
Overall Assessment
This is an intellectually ambitious and technically thorough paper that opens a genuinely new avenue for addressing one of particle physics' deepest questions. The theoretical framework is sound and the experimental projections, while optimistic, are grounded in realistic detector capabilities. The primary limitation—dependence on astrophysical source models and distant-future detector networks—is honestly acknowledged. The work is likely to stimulate significant follow-up in both theoretical and experimental communities.
Generated Jun 17, 2026
Comparison History (20)
Paper 1 addresses one of the most fundamental open questions in particle physics—the Dirac vs. Majorana nature of neutrinos—with a novel, complementary approach that bypasses known blind spots in neutrinoless double beta decay. It connects two experimental frontiers (beam-dump experiments and neutrino telescopes) in an innovative way, with broad implications across particle physics, astrophysics, and cosmology. Paper 2 is methodologically clever and practically important for DUNE, but its scope is more narrowly focused on cross-section calibration. Paper 1's potential to resolve a foundational question gives it higher impact.
Paper 2 addresses a fundamental open question in particle physics: whether neutrinos are Dirac or Majorana fermions. While Paper 1 offers a valuable, practical calibration technique for DUNE's supernova detection, Paper 2 proposes a highly novel, multi-experiment probe combining beam-dump and astrophysical neutrino telescopes. By bypassing the traditional blind spots of neutrinoless double beta decay searches, this method offers a new pathway to resolve the fundamental nature of neutrino mass, promising significantly broader theoretical and experimental impacts across particle physics, astrophysics, and cosmology.
Paper 1 addresses one of the most fundamental open questions in particle physics—the Dirac vs. Majorana nature of neutrinos—with a novel, complementary approach that bypasses the blind spots of neutrinoless double beta decay. It connects two experimental frontiers (beam-dump experiments and neutrino telescopes) in a correlated strategy, offering broader impact across particle physics and astrophysics. Paper 2 proposes a creative axion detection method using Weyl semimetals, but targets a more incremental advance in detector concepts. Paper 1's novelty, breadth of impact, and potential to resolve a foundational question give it higher scientific impact.
Paper 2 has higher potential impact: it addresses a central open question (Dirac vs Majorana neutrinos) with a novel, testable strategy combining terrestrial (SHiP beam-dump) and astrophysical (TeV–PeV neutrino flavor ratios) measurements, offering broad relevance to particle physics, cosmology, and neutrino astronomy. Its proposed correlated signals/null tests could decisively complement or bypass neutrinoless double beta decay blind spots, with clear real-world experimental pathways and timely alignment with upcoming facilities. Paper 1 is technically rigorous but more specialized, mainly refining TMD/CSS evolution in specific gauges.
Paper 2 addresses a fundamental and long-standing open question in physics (whether neutrinos are Dirac or Majorana) by proposing a novel, testable multi-experiment approach. Its ability to bypass existing experimental blind spots using near-future facilities gives it broad relevance and high potential for groundbreaking discovery, whereas Paper 1 provides a more specialized, albeit rigorous, theoretical framework for specific axion models.
Paper 2 addresses the fundamental and long-standing problem of hadronization in QCD through a novel quantum-information-inspired framework, connecting quantum decoherence to the Lund string model. It explains existing experimental data from RHIC and LHC, establishing a quantitative link between QCD vacuum structure, entanglement, and decoherence. This interdisciplinary approach bridging QCD and quantum information science has broad implications. Paper 1 proposes an interesting complementary probe for neutrino nature but relies on future experiments (SHiP, next-gen telescopes) with uncertain timelines, whereas Paper 2 already explains measured data, giving it more immediate and demonstrable impact.
Paper 1 offers a more novel, testable strategy to resolve the Dirac vs Majorana neutrino question by combining fixed-target HNL searches (SHiP) with flavor-ratio measurements of TeV–PeV astrophysical neutrinos, explicitly addressing key blind spots of neutrinoless double beta decay. It has clear, near-term experimental pathways and a potentially decisive outcome with broad implications for particle physics, cosmology, and neutrino astronomy. Paper 2 is timely and useful for guiding axion model-building/experiment priorities, but it is more of a naturalness/UV-quality assessment and is less directly discovery-enabling.
Determining whether neutrinos are Dirac or Majorana is one of the most profound open questions in particle physics. Paper 2 proposes a novel, testable approach using upcoming experiments to bypass current blind spots in neutrinoless double beta decay. Its potential to definitively answer this fundamental question offers massive scientific impact. In contrast, Paper 1 establishes limits on thermal solar neutrinos but acknowledges being far from an actual detection, making its near-term impact more incremental.
Paper 1 addresses one of the most profound open questions in particle physics: whether neutrinos are Majorana or Dirac fermions. By proposing a novel, testable method that bypasses the limitations of neutrinoless double beta decay, it offers a realistic path to a fundamental discovery. In contrast, Paper 2 explores a highly exotic phenomenological scenario (a periodically varying Z' mass) which, while methodologically innovative, has a much narrower theoretical motivation and lower probability of fundamentally altering our understanding of the Standard Model.
Paper 1 targets a foundational SM question (Dirac vs Majorana neutrinos) with a novel, complementary strategy that avoids 0νββ blind spots by leveraging flavor-shift signatures from HNLs in both SHiP and astrophysical neutrino telescopes. The correlated multi-experiment test is timely and could deliver a definitive qualitative outcome (prove Majorana or strongly favor Dirac), with broad impact across particle physics, neutrino phenomenology, and astrophysics. Paper 2 is innovative and timely for LISA-era ALP searches, but it is a sensitivity-projection/technique extension with less decisive interpretability and narrower cross-field consequences.