A null search can set a useful limit without deciding the deeper question that motivated it. KamLAND-Zen reports no neutrinoless double-beta signal excess in its complete dataset. That result constrains the decay's half-life at the experiment's sensitivity; it does not establish that neutrinos are Dirac particles or rule out Majorana neutrinos.
The distinction matters because a measured non-detection and a particle-nature verdict are different claims. The limit depends on the search and its statistical construction. Turning it into an effective Majorana-mass bound requires additional physical and nuclear-model assumptions.
What the combined search reports
For its combined KamLAND-Zen 400+800 datasets, the collaboration reports a 90% confidence lower limit of years. This is the combined result. The separate KamLAND-Zen 800-only analysis reports limits of years using a profile-likelihood construction and years using Feldman–Cousins; those are different statistical results for a different dataset combination.
The collaboration also gives an effective-mass upper-limit range of – meV at 90% confidence. This translation assumes dominant light-Majorana-neutrino exchange and uses stated choices for the axial coupling, phase-space factor and nuclear matrix elements. The spread across listed nuclear calculations is part of the result's interpretation: it is not a measured mass interval or a model-independent mass determination.
What oscillations do and do not show
Neutrino oscillation probabilities do not depend on the Majorana phase matrix in the three-neutrino framework used by NuFIT. The effective mass relevant to neutrinoless double-beta decay instead combines the squared electron-row mixing elements with the neutrino masses, so relative phases can affect the sum through interference.
The bounded source review found an apparent sign mismatch between NuFIT's displayed mixing-matrix convention and its later expanded effective-mass equation when the same phase labels are used in both. The paper does not state that the labels are being changed between those equations. Since the free phases span the same full effective-mass envelope after relabelling, the notation issue does not change the oscillation result or that envelope. The invariant effective-mass definition remains the clearest way to state the quantity.
The boundary of the conclusion
A null result at finite sensitivity does not distinguish Dirac from Majorana neutrinos. The quoted effective-mass range is conditional on the light-Majorana-exchange mechanism and the nuclear calculations used; this source review does not evaluate other lepton-number-violating mechanisms.
The review compares published sources. It does not reproduce an experimental fit, detector-data analysis, likelihood, nuclear calculation or mass-ordering test, and it is not an exhaustive literature review. No human specialist or peer review of this synthesis is claimed. It establishes no new experimental result, field-level novelty or priority claim.
Sources and review limits
The underlying bounded source comparison, including its locators and limitations, is recorded in the Research Artifact Ledger's Dirac/Majorana null-search review at the reviewed commit. The primary sources are the KamLAND-Zen Collaboration's complete-dataset paper and arXiv version 2, and Esteban et al.'s NuFIT 6.0 paper, version 2.
This article is an explanatory account of that bounded source review, not a new particle-physics result. AI assisted with source analysis and drafting; the author remains responsible for the claims and their limits. The featured image is generated conceptual artwork, not a scientific figure or data. No source data, table or figure is reproduced.
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