Inside three of the world’s deepest underground laboratories, three different experimental teams are listening for a process that might happen, on average, once per nucleus per 10²⁶ years — about ten thousand trillion trillion years, vastly longer than the age of the universe.
The process is called neutrinoless double-beta decay, or 0νββ. If it exists at all, observing it would prove that neutrinos are their own antiparticles — that they are Majorana fermions — and would rewrite half a century of particle physics theory. It would almost certainly bring another Nobel Prize.
Three of the largest active experiments — GERDA (now succeeded by LEGEND) at Gran Sasso in Italy, and KamLAND-Zen in the Kamioka mine in Japan — have been steadily improving sensitivity for years. None has seen the decay yet. None has ruled it out either.
What 0νββ would look like
In ordinary double-beta decay, two neutrons in a nucleus simultaneously turn into two protons, emitting two electrons and two antineutrinos in the process. This is rare but observable: physicists have measured ordinary double-beta decay in nuclides like germanium-76, xenon-136, and tellurium-130, with half-lives around 10²¹ years.
Neutrinoless double-beta decay, if it occurred, would look almost the same — except no neutrinos would come out. The two electrons would carry away the full available energy of the decay, producing a sharp spectral line at the endpoint energy of the decay rather than the smooth distribution that ordinary double-beta decay produces.
For 0νββ to happen, the two emitted antineutrinos from the underlying weak vertices must annihilate each other. That can only occur if the antineutrino is also a neutrino — that is, if neutrinos are Majorana particles. Find a line at the endpoint, and you have proven the Majorana hypothesis. Find no line, and you push the half-life lower bound further out.
How you actually search for it
Three challenges shape every 0νββ experiment.
You need an enormous mass of the target isotope. With half-lives expected to be at least 10²⁶ years per nucleus, you cannot just measure one or two nuclei. You need tons. Each experiment chooses an isotope, accumulates the enriched material in usable form, and packs the detector with as much of it as possible.
You need to be deep underground. Cosmic rays produce gamma rays and neutrons that can mimic 0νββ signals. The deeper you go, the more rock shields the detector. Gran Sasso (1.4 km of rock above), Kamioka (1 km), SNOLAB (2 km), and Sanford Lab (1.5 km) all host 0νββ experiments.
You need radio-pure materials. Trace contamination from uranium, thorium, or potassium-40 in the detector materials themselves can produce decays at energies near the 0νββ region of interest. Every screw, cable, and crystal has to be vetted with extraordinary care. Some experiments build their detectors from materials processed before the atmospheric atomic-bomb tests of the mid-twentieth century, because postwar materials carry low but measurable contamination from fallout.
The three approaches
GERDA and LEGEND use germanium-76, enriched to high isotopic purity. The germanium itself is the detector: arrays of ultra-pure germanium crystals are immersed in a tank of liquid argon at cryogenic temperatures. Decays inside the crystals produce charge signals that the readout electronics record. GERDA ran from 2011 to 2019 and set world-leading limits. LEGEND-200, its direct successor, has been taking data since 2023 with about a ton of germanium. LEGEND-1000, the next stage, is being designed to reach about a ton of germanium with an order of magnitude better background rejection. The current lower limit on the germanium-76 0νββ half-life is about 1.8 × 10²⁶ years.
KamLAND-Zen uses xenon-136 dissolved in liquid scintillator. The xenon — about 750 kg of enriched isotope — sits in a balloon at the center of the much larger KamLAND scintillator detector. Decays inside the balloon produce light that the surrounding photomultipliers detect. KamLAND-Zen 800 has been running since 2019 and reported a lower limit on the xenon-136 0νββ half-life of around 2.3 × 10²⁶ years — the most stringent limit on any isotope to date. The KamLAND2-Zen upgrade aims to reach about 10²⁷ years.
Other experiments focus on different isotopes: CUORE uses tellurium-130 with a cryogenic bolometer technique, EXO/nEXO uses xenon-136 in a liquid-xenon TPC, CUPID uses molybdenum-100 with scintillating bolometers. Each isotope has different nuclear physics, different backgrounds, and different systematic uncertainties. Multiple isotopes are valuable because any positive signal needs cross-confirmation; finding 0νββ in one isotope but not another would be deeply suspicious.
What the limits already tell us
Even without finding 0νββ, the existing limits constrain the effective Majorana mass — a particular combination of neutrino masses and oscillation parameters that 0νββ experiments directly access. The current effective Majorana mass upper bound is roughly 150 meV, depending on which nuclear matrix element calculation you trust.
This number is significant for two reasons.
It is comparable to the direct neutrino mass limit from KATRIN (about 450 meV) and pushing into the same physically interesting regime. The next generation of 0νββ experiments will probe effective masses around 10–50 meV — well into the range where the inverted-ordering scenario predicts a definite signal.
It places real constraints on theory. The simplest see-saw models with normal mass ordering predict effective Majorana masses in the few-meV range — below current experiments but within reach of LEGEND-1000 and KamLAND2-Zen. The simplest see-saw models with inverted ordering predict effective masses in the 15–50 meV range — squarely within reach. If we still don’t find 0νββ by the early 2030s, the simplest Majorana scenarios will be in serious tension.
What’s at stake
If 0νββ is observed, three things become certain simultaneously.
Neutrinos are Majorana. The hypothesis Ettore Majorana wrote down in his last paper would be confirmed for the first time, almost a century later.
Lepton number is not conserved. A Standard Model assumption that has held for fifty years would be broken.
Leptogenesis becomes plausible. The scenario that explains the matter-antimatter asymmetry of the universe via heavy Majorana neutrinos would gain a major experimental pillar.
The reverse is also significant. If 0νββ is not found over the next decade, the inverted-ordering scenario with Majorana neutrinos becomes very hard to maintain, and the Dirac picture gains weight.
The hunt continues. Three experiments are running. The next decade should give a definitive answer.
For the Majorana hypothesis at the heart of the search, see Majorana or Dirac?. For the man who first proposed Majorana fermions, see Ettore Majorana. For why this matters for cosmology, see Leptogenesis.
Further reading
Primary sources
- Agostini et al. (GERDA), “Final Results of GERDA on the Search for Neutrinoless Double-β Decay”, Phys. Rev. Lett. 125:252502 (2020) — the final ⁷⁶Ge result
- Abe et al. (KamLAND-Zen), “Search for Majorana Neutrinos near the Inverted Mass Hierarchy Region with KamLAND-Zen”, Phys. Rev. Lett. 130:051801 (2023) — the leading ¹³⁶Xe limit
- Abgrall et al. (LEGEND), “The Large Enriched Germanium Experiment for Neutrinoless Double Beta Decay (LEGEND)”, 2021 — the design paper for the next generation
- Adams et al. (nEXO), “nEXO: neutrinoless double beta decay search beyond 10²⁸ year half-life sensitivity”, J. Phys. G 49:015104 (2022)
Background and context
- M. J. Dolinski, A. W. P. Poon, W. Rodejohann, “Neutrinoless Double-Beta Decay: Status and Prospects”, Ann. Rev. Nucl. Part. Sci. 69:219 (2019) — comprehensive review
- Particle Data Group — Neutrinoless Double-β Decay review — canonical reference
- Wikipedia: Neutrinoless double beta decay
Frequently asked
What is neutrinoless double-beta decay?
A hypothetical radioactive decay in which two neutrons in a nucleus simultaneously turn into protons and emit two electrons — with no accompanying neutrinos. Observation would prove that neutrinos are Majorana fermions (their own antiparticles). It has never been detected; the predicted half-lives are at least 10²⁶ years per nucleus.
Which experiments are racing to detect it?
The three leading ton-scale programs: LEGEND (germanium-76 at Gran Sasso and SNOLAB), KamLAND-Zen (xenon-136 at Kamioka), and nEXO (xenon-136 at SNOLAB). Smaller-scale programs include CUORE/CUPID (tellurium, molybdenum) at Gran Sasso, NEXT-100 (gas xenon) at Canfranc, and the completed Majorana Demonstrator at SURF.
What's the current best limit?
KamLAND-Zen 800 has set the world's most stringent limit at about 2.3 × 10²⁶ years for xenon-136, corresponding to an effective Majorana neutrino mass below roughly 36-156 meV depending on the nuclear matrix element calculation used. Limits from LEGEND-200 in germanium-76 are comparable.
Why does it matter?
Because detection would establish three things at once: neutrinos are Majorana, lepton number is not conserved, and leptogenesis (the leading scenario for the matter-antimatter asymmetry) gains experimental support. A null result by 2032 would essentially exclude the inverted-ordering Majorana scenario, pointing either to Dirac neutrinos or to normal ordering with very small effective mass.
When might a discovery happen?
If neutrinos are Majorana with inverted mass ordering, LEGEND-1000, KamLAND2-Zen, or nEXO should produce a detectable signal by the early 2030s. Confirmation across multiple isotopes would be essential. If no signal is found by then, the broader theoretical landscape will need significant revision.
Cite this article 5 formats
APA
Neutrino Times Editorial Team. (2025, September 7). The hunt for neutrinoless double-beta decay: GERDA, LEGEND, KamLAND-Zen. Neutrino Times. https://neutrino-times.com/articles/neutrinoless-double-beta-decay-hunt-gerda-legend-kamland-zen/
Chicago
Neutrino Times Editorial Team. "The hunt for neutrinoless double-beta decay: GERDA, LEGEND, KamLAND-Zen." Neutrino Times, September 7, 2025. https://neutrino-times.com/articles/neutrinoless-double-beta-decay-hunt-gerda-legend-kamland-zen/.
MLA
Neutrino Times Editorial Team. "The hunt for neutrinoless double-beta decay: GERDA, LEGEND, KamLAND-Zen." Neutrino Times, 7 Sep. 2025, https://neutrino-times.com/articles/neutrinoless-double-beta-decay-hunt-gerda-legend-kamland-zen/.
BibTeX
@misc{neutrino-times-neutrinoless-double-beta-decay-hunt-gerda-legend-kamland-zen,
author = {Neutrino Times Editorial Team},
title = {The hunt for neutrinoless double-beta decay: GERDA, LEGEND, KamLAND-Zen},
howpublished = {Neutrino Times},
year = {2025},
month = {sep},
url = {https://neutrino-times.com/articles/neutrinoless-double-beta-decay-hunt-gerda-legend-kamland-zen/},
note = {Accessed: 2025-09-07}
} RIS
TY - GEN TI - The hunt for neutrinoless double-beta decay: GERDA, LEGEND, KamLAND-Zen AU - Neutrino Times Editorial Team PY - 2025 DA - 2025-09-07 PB - Neutrino Times UR - https://neutrino-times.com/articles/neutrinoless-double-beta-decay-hunt-gerda-legend-kamland-zen/ ER -