This is the fifth part of the 2030s Roadmap series. We turn to the experiments hunting for the most fundamental physics question in neutrino physics: is the neutrino its own antiparticle?
What 0νββ is and why it’s the test
Neutrinoless double beta decay (0νββ) is a hypothetical process: $2n \to 2p + 2e^-$ in a nucleus, with no neutrinos in the final state. It can occur only if neutrinos are Majorana — their own antiparticles.
If neutrinos are Majorana, the rate of 0νββ depends on:
- The effective Majorana mass $m_{\beta\beta}$ (a coherent sum over neutrino masses weighted by PMNS elements and Majorana phases).
- Nuclear matrix elements (calculable from nuclear structure theory, with substantial uncertainty).
- The half-life of the decay scales inversely with $m_{\beta\beta}^2$.
The mass ordering and absolute mass scale set boundaries on what $m_{\beta\beta}$ can be:
- Inverted ordering: $m_{\beta\beta} > 15$ meV approximately.
- Normal ordering: $m_{\beta\beta}$ can range from essentially 0 (with cancellation between Majorana phases) up to ~5 meV.
The “ton-scale” generation is designed to fully cover the inverted-ordering region (10-15 meV $m_{\beta\beta}$) and partially cover the normal-ordering band.
The current generation status
As of 2026, the leading constraints come from:
- KamLAND-Zen (¹³⁶Xe): half-life > 2.3 × 10²⁶ yr, $m_{\beta\beta}$ < 36-156 meV.
- LEGEND-200 / GERDA (⁷⁶Ge): half-life > 1.8 × 10²⁶ yr.
- CUORE (¹³⁰Te): half-life > 3.2 × 10²⁵ yr.
- EXO-200 (¹³⁶Xe): half-life > 3.5 × 10²⁵ yr.
The wide range in $m_{\beta\beta}$ constraints reflects nuclear-matrix-element uncertainties — different theoretical calculations give different translations from half-life to mass.
The three ton-scale flagships
LEGEND-1000: 1,000 kg of germanium-76 enriched to ~88% in high-purity germanium (HPGe) crystal arrays. The crystals function as the detector and the isotope source simultaneously. Background suppression through ultra-low-radioactivity construction and pulse-shape discrimination of source signal from gamma backgrounds. Target sensitivity: half-life $\sim 10^{28}$ years, $m_{\beta\beta} \sim 10-15$ meV.
nEXO: 5,000 kg of xenon-136 (enriched to ~90%) in a single-phase liquid-xenon time projection chamber. Detector reads ionization and scintillation simultaneously, distinguishing single-site decay events from multi-site gamma backgrounds. Located at SNOLAB. Target sensitivity: half-life $\sim 10^{28}$ years, $m_{\beta\beta} \sim 10$ meV.
KamLAND2-Zen: Upgrade of the current KamLAND-Zen, increasing the xenon-loaded scintillator mass to approximately 1 ton of ¹³⁶Xe. Located at Kamioka. Target sensitivity similar to LEGEND-1000 and nEXO.
Supporting programs
CUPID (Cryogenic Underground Particle and Imaging Detector): Successor to CUORE. About 240 kg of enriched Li₂¹⁰⁰MoO₄ scintillating bolometers. The simultaneous heat and light readout discriminates 0νββ from background. Located at Gran Sasso. Target: $m_{\beta\beta} \sim 10-15$ meV.
SNO+: Tellurium-130 in liquid scintillator at SNOLAB. Currently loaded with ~1,300 kg of Te-130. Different systematics from other experiments. Target similar.
NEXT: Gas xenon TPC at the Canfranc Laboratory in Spain. Provides topological imaging that no liquid TPC can match.
How the ton-scale is different
The key difference from current-generation experiments is scale. With 1 ton of isotope and 5-10 years of running, expected sensitivities are sufficient to either detect 0νββ if the mass is in the IO band, or to robustly rule out IO if no signal is seen.
The technical challenges:
- Backgrounds: Need to suppress to below the level of expected 0νββ signal. Each experiment uses a different combination of: deep underground location, ultra-low-radioactivity materials, particle-identification techniques.
- Energy resolution: The 0νββ signature is a single peak at the Q-value, with no continuous background. High energy resolution makes the search easier.
- Topological discrimination: Some experiments (NEXT-100, SNO+) use the multi-track topology of two-electron events to distinguish from gamma-ray backgrounds.
Timeline
2026-2028: Final commissioning of CUPID and SNO+ at full tellurium load. First competitive ton-scale results from LEGEND-200 (precursor to LEGEND-1000).
2028-2032: Construction of LEGEND-1000 and nEXO. KamLAND-Zen upgrade. First results from CUPID.
2032-2038: Full ton-scale operations. If 0νββ exists in the IO band, discovery happens here.
Late 2030s: Combined fits across multiple experiments give the strongest possible constraints.
What different outcomes would mean
Discovery at $m_{\beta\beta} \sim 15$ meV: Inverted ordering confirmed via 0νββ. Majorana nature confirmed. Seesaw mechanism heavily favored. Leptogenesis becomes plausible.
Discovery at $m_{\beta\beta} \sim 5$ meV (below IO band): Normal ordering. The lightest neutrino must have substantial mass. Phase cancellations probably not too severe.
Non-detection at full ton-scale sensitivity: Either Dirac neutrinos, or normal ordering with strong Majorana-phase cancellations. Either is informative; the field would need to design even more ambitious next-generation experiments to push deeper into the normal-ordering band.
What’s beyond ton-scale
If the ton-scale doesn’t see 0νββ, the next step is multi-ton or even larger detectors. Several concepts are under study:
- Theia (large-scale Cherenkov-scintillator hybrid).
- DARWIN (50-ton xenon detector, primarily dark-matter, with 0νββ secondary).
- JUNO could potentially be reloaded with isotope.
These are 2040s programs.
The big-picture significance
The ton-scale 0νββ generation is the most direct test of the Majorana hypothesis ever attempted. The result will have enormous implications:
- For leptogenesis and the matter-antimatter asymmetry of the universe.
- For seesaw mechanism and the origin of neutrino mass.
- For the basic structure of the Standard Model and beyond.
The next part of this series turns to the cosmological-scale experiment that complements all of this: CMB-S4 and the cosmological neutrino-mass measurement.
Frequently asked
What does 'ton-scale' mean?
Detectors with about 1,000 kg (1 metric ton) of the isotope being studied — substantially larger than the current generation (hundreds of kg at most). The scale is set by the sensitivity required to fully cover the inverted-ordering Majorana-mass band, where the effective Majorana mass m_ββ is bounded below by about 15 meV.
What are the three flagship ton-scale experiments?
LEGEND-1000 (1,000 kg of germanium-76 in HPGe arrays, US/Europe), nEXO (5,000 kg of xenon-136 in a liquid TPC, Canada), KamLAND2-Zen (upgrade to ~1 ton of xenon-136 in Kamioka liquid scintillator, Japan). Plus the CUPID program (240 kg of enriched Li₂¹⁰⁰MoO₄ scintillating bolometers, Italy) and SNO+ (Te-130 in scintillator) at slightly smaller scale.
When does discovery happen if neutrinos are Majorana?
If the mass ordering is inverted and the absolute mass is near the cosmology upper bound, current-generation experiments could detect 0νββ. If the ordering is normal and the lightest mass is small, ton-scale is needed. The most likely scenario for first detection: 2032-2038 at one or more of the ton-scale experiments.
What if no signal is seen?
Then either neutrinos are Dirac (no 0νββ regardless of mass), or the mass ordering is normal with very small m_ββ (within the natural cancellation region where Majorana phases can suppress 0νββ below ~1 meV). Either result would be major information about the structure of the neutrino sector.
Cite this article 5 formats
APA
Neutrino Times Editorial Team. (2026, April 28). The 2030s Roadmap — Part 5: The 0νββ ton-scale — LEGEND-1000, nEXO, KamLAND2-Zen. Neutrino Times. https://neutrino-times.com/articles/roadmap-2030s-part-5-tonscale-0nbb/
Chicago
Neutrino Times Editorial Team. "The 2030s Roadmap — Part 5: The 0νββ ton-scale — LEGEND-1000, nEXO, KamLAND2-Zen." Neutrino Times, April 28, 2026. https://neutrino-times.com/articles/roadmap-2030s-part-5-tonscale-0nbb/.
MLA
Neutrino Times Editorial Team. "The 2030s Roadmap — Part 5: The 0νββ ton-scale — LEGEND-1000, nEXO, KamLAND2-Zen." Neutrino Times, 28 Apr. 2026, https://neutrino-times.com/articles/roadmap-2030s-part-5-tonscale-0nbb/.
BibTeX
@misc{neutrino-times-roadmap-2030s-part-5-tonscale-0nbb,
author = {Neutrino Times Editorial Team},
title = {The 2030s Roadmap — Part 5: The 0νββ ton-scale — LEGEND-1000, nEXO, KamLAND2-Zen},
howpublished = {Neutrino Times},
year = {2026},
month = {apr},
url = {https://neutrino-times.com/articles/roadmap-2030s-part-5-tonscale-0nbb/},
note = {Accessed: 2026-04-28}
} RIS
TY - GEN TI - The 2030s Roadmap — Part 5: The 0νββ ton-scale — LEGEND-1000, nEXO, KamLAND2-Zen AU - Neutrino Times Editorial Team PY - 2026 DA - 2026-04-28 PB - Neutrino Times UR - https://neutrino-times.com/articles/roadmap-2030s-part-5-tonscale-0nbb/ ER -