---
title: 'LEGEND-200: Neutrinoless Beta Decay Experiment'
url: https://www.emergentmind.com/topics/legend-200
type: topic
---

# LEGEND-200: Neutrinoless Beta Decay Experiment

Searching arXiv for LEGEND-200 and related technical papers.
LEGEND-200 is the first operational phase of the Large Enriched Germanium Experiment for Neutrinoless $\beta\beta$ Decay, a staged $^{76}$Ge program that combines the low-background methods of GERDA with the detector-development and radiopurity practices of the MAJORANA Demonstrator. It searches for neutrinoless double beta decay by operating enriched high-purity germanium detectors as bare diodes in instrumented liquid argon at the Laboratori Nazionali del Gran Sasso, and its first blinded physics result established the initial performance and half-life reach of the LEGEND program [2509.21166].

## 1. Physics objective and decay signature

LEGEND-200 is designed to search for the decay
\[
^{76}\mathrm{Ge} \rightarrow {}^{76}\mathrm{Se} + 2e^-,
\]
with
\[
Q_{\beta\beta}=2039.061(7)\ \mathrm{keV}.
\]
In the Standard Model, ordinary two-neutrino double beta decay is allowed, whereas neutrinoless double beta decay is forbidden. Observation of the neutrinoless mode would therefore demonstrate that neutrinos are Majorana fermions and establish lepton-number violation by two units, $\Delta L = 2$ [2509.21166].

In the light-Majorana-exchange picture, the inverse half-life is written as
\[
(T_{1/2}^{0\nu})^{-1} = G^{0\nu} |M^{0\nu}|^2 \frac{m_{\beta\beta}^2}{m_e^2},
\]
so an experimental limit on $T_{1/2}^{0\nu}$ constrains the effective Majorana mass $m_{\beta\beta}$ [2508.18573]. The process is also discussed in connection with leptogenesis and the matter–antimatter asymmetry in the Universe [1905.06572].

The germanium approach is central to the LEGEND strategy because the detector material is simultaneously source and detector, the isotope can be enriched to high $^{76}$Ge fraction, and HPGe technology provides energy resolution around $0.1\%$ FWHM at $Q_{\beta\beta}$ together with pulse-shape sensitivity to event topology [2509.21166].

## 2. Position within the LEGEND roadmap

LEGEND is a two-stage program. LEGEND-200 is the current-generation phase, while LEGEND-1000 is the later tonne-scale phase. The design goal for LEGEND-200 is a $3\sigma$ discovery sensitivity for a $0\nu\beta\beta$ half-life of about $10^{27}$ yr after $1$ ton yr exposure at a target background index of
\[
2\cdot 10^{-4}\ \text{counts/(keV kg yr)},
\]
whereas LEGEND-1000 aims at sensitivity beyond $10^{28}$ yr [2509.21166].

This program was formulated by explicitly building on GERDA and the MAJORANA Demonstrator. GERDA had already demonstrated background-free operation at current exposures with a background index near $Q_{\beta\beta}$ of about $5.6\text{–}5.7\times10^{-4}$ cts/(keV·kg·yr), while the MAJORANA Demonstrator achieved an energy resolution of $2.53\pm0.08$ keV FWHM at $Q_{\beta\beta}=2039$ keV [1905.06572]. A later review framed LEGEND-200 as a 200 kg deployment in the GERDA facility at LNGS with a target sensitivity of about $10^{27}$ yr at 90% C.L. with 1 t·yr exposure and a background reduction of roughly a factor of three beyond GERDA’s final level [2109.07575].

A major enabling development for LEGEND-200 is the inverted-coaxial point-contact detector. In GERDA, five enriched IC detectors manufactured from material enriched to 87.7(5)% in $^{76}$Ge reached resolutions at $Q_{\beta\beta}$ of about 2.1 keV FWHM in vacuum cryostat, and after 18 months of operation in liquid argon achieved a background index after analysis cuts of $4.9^{+7.3}_{-3.4}\times 10^{-4}$ counts/(keV·kg·yr) around $Q_{\beta\beta}$, establishing their feasibility for LEGEND [2103.15111].

## 3. Apparatus and detector concept

After a commissioning period with 60 kg of Ge, LEGEND-200 began physics data taking in March 2023 at LNGS with a total Ge mass of 142.5 kg. In the first deployment, four detector geometries were used: 6 Coax and 28 BEGe detectors inherited from GERDA, 26 PPC detectors from the MAJORANA Demonstrator, and 41 newly produced inverted coaxial point-contact detectors. The detectors are enriched in $^{76}$Ge beyond 86%, with enrichment fractions between 86% and 92% depending on detector type, and crystal masses range from roughly $0.5$ to $4\ \mathrm{kg}$ [2509.21166].

The detectors are operated as bare diodes in a $64\ \mathrm{m}^3$ liquid argon cryostat at about $88\ \mathrm{K}$. The argon acts simultaneously as coolant, passive shield, and active veto medium. The active veto uses wavelength-shifting fiber barrels read out by SiPMs to detect argon scintillation light and provide Ge–LAr anticoincidence. Outside the cryostat is a $590\ \mathrm{m}^3$ ultra-pure water tank instrumented with PMTs, which provides additional passive shielding and functions as a Cherenkov muon veto. The LNGS overburden of 1400 m of rock, corresponding to almost 3500 m water equivalent, suppresses the cosmic muon flux to about $1.2$ muons/$(\mathrm{m}^2\ \mathrm{h})$ [2509.21166].

The liquid-argon light instrumentation is itself a dedicated subsystem. Its front-end electronics, installed in July 2021, were designed to read out the SiPM response to argon scintillation while satisfying stringent radiopurity constraints and achieved a very low overall level of electrical noise of $250\ \mu$V peak-to-peak [2211.03069]. Near the detectors, further background control is implemented through low-background PEN support structures: molded PEN components were qualified as active holders with a light yield of $(5440 \pm 220)\ \text{photons/MeV}e^-$ and an estimated self-veto threshold of roughly 50–250 keV depending on position [2204.13747].

Support infrastructure around the detector array was also optimized for calibration and materials control. Custom low-neutron-emission $^{228}$Th calibration sources were produced for LEGEND-200, with a measured neutron flux of
\[
\left( 4.27 \pm 0.60_{\rm stat} \pm 0.92_{\rm syst} \right) \times 10^{-4}\ \text{n/(kBq·s)},
\]
approximately one order of magnitude below that of commercial sources, making the neutron-induced background rate negligible compared to other background sources in LEGEND-200 [2211.05026]. Detector production for the program required dedicated enriched-germanium processing; one campaign reported hydrogen reduction of a batch of 23 kg isotopically enriched Ge with an average yield of 99.85%, followed by zone refining yielding an overall Ge yield of 99.05% [2009.07585].

## 4. Calibration, reconstruction, and blinded analysis

LEGEND-200 relies on weekly $^{228}$Th source calibrations to monitor the energy scale, resolution, and detector stability. A dedicated calibration-and-performance analysis reported a combined average resolution of $(2.47 \pm 0.08)$ keV at
\[
Q_{\beta\beta} = 2039~\mathrm{keV},
\]
with weekly variation of calibration peak positions below $0.05$ keV for energies up to 2614.5 keV. The same study reported explicit corrections for residual non-linearities and energy bias in the region of interest [2605.22479].

The first $0\nu\beta\beta$ search used a strictly blind analysis in the region
\[
Q_{\beta\beta} \pm 25\ \mathrm{keV}.
\]
For statistical extraction of the background and the half-life limit, the fit window was
\[
[1930,2190]\ \mathrm{keV},
\]
excluding the known $\gamma$ lines at $[2104\pm5]$ keV from $^{208}\mathrm{Tl}$ and $[2119\pm5]$ keV from $^{214}\mathrm{Bi}$ [2509.21166].

Event selection combined multiple rejection layers. The muon veto and Ge-detector multiplicity cut each have efficiencies above 99.9% at $Q_{\beta\beta}$. The liquid-argon veto reaches
\[
\epsilon_{\mathrm{LAr}(Q_{\beta\beta})} \sim 93\%,
\]
and pulse-shape discrimination has
\[
\epsilon_{\mathrm{PSD}(Q_{\beta\beta})} \sim 76\text{–}85\%
\]
depending on detector type. This sequence strongly suppresses Compton-scattered $\gamma$ backgrounds and other non-signal topologies [2509.21166].

The data were partitioned into two categories with different expected background conditions. The “golden dataset” comprises 48.3 kg yr from BEGe, PPC, and Mirion-produced ICPC detectors. The “silver dataset” comprises 12.7 kg yr from Coax and ORTEC-produced ICPC detectors [2509.21166].

Offline waveform-quality control is also an explicit component of the LEGEND-200 analysis chain. A semi-supervised Affinity Propagation plus Support Vector Machine method developed on the Full Chain Test is reported as being used to accelerate data-cleaning development for LEGEND-200 and as the main data cleaning method in the Juleana secondary software stack; the quoted maximum sacrifice of physics events is
\[
0.024 ^{+0.004}_{-0.003}\%.
\]
[2410.14701]

## 5. First-year physics result

In about one year of running, LEGEND-200 accumulated a total physics exposure of 61 kg yr usable for the $0\nu\beta\beta$ analysis [2509.21166]. After all cuts, 11 events remained in the analysis window: 7 in the golden dataset and 4 in the silver dataset. The measured background indices were
\[
0.5^{+0.3}_{-0.2} \cdot 10^{-3}\ \text{counts/(keV kg yr)}
\]
for the golden dataset and
\[
1.3^{+0.8}_{-0.5} \cdot 10^{-3}\ \text{counts/(keV kg yr)}
\]
for the silver dataset [2509.21166].

The statistical interpretation was carried out in a frequentist framework, with a Bayesian analysis reported as giving compatible results. No evidence for a $0\nu\beta\beta$ signal was found, and the first LEGEND-200 lower limit was
\[
T_{1/2}^{0\nu} > 0.5 \cdot 10^{26}\ \mathrm{yr}
\]
at 90% C.L. The median exclusion sensitivity was
\[
1.0\cdot10^{26}\ \mathrm{yr}.
\]
Using ranges of phenomenological nuclear matrix elements, the corresponding upper limit on the effective Majorana mass was quoted as
\[
m_{\beta\beta} < 75\text{–}200\ \mathrm{meV}.
\]
[2509.21166]

The collaboration also performed a combined analysis with the preceding germanium-based experiments. A joint fit to GERDA (127.2 kg yr), MAJORANA Demonstrator (64.5 kg yr), and LEGEND-200 (61 kg yr) found no signal and yielded
\[
T_{1/2}^{0\nu} > 1.9 \cdot 10^{26}\ \mathrm{yr}
\]
at 90% C.L., with a median exclusion sensitivity of
\[
2.8\cdot 10^{26}\ \mathrm{yr}.
\]
A conference-style summary emphasized that this combined result provided the best exclusion sensitivity yet achieved in any $0\nu\beta\beta$ search [2509.21166; 2508.18573].

## 6. Background excess, detector subsets, and longer-term significance

A central technical result of the first year was that the observed background was somewhat higher than expected from prior radioassay-based predictions, including in the region around $Q_{\beta\beta}$. According to the first-results paper, the background-model fit to the less selective spectrum suggested that the excess could be attributable to contamination from the $^{228}\mathrm{Th}$ chain. This motivated a screening and cleaning campaign and a redeployment of the array in May 2025, with post-redeployment data expected to verify the background reduction [2509.21166].

The two analysis categories encapsulate an important detector-performance distinction. The lower background in the golden dataset reflects the better-performing detector classes and cleaner subset, while the silver dataset contains the detector populations associated with a higher residual rate [2509.21166]. The same pattern appears in energy performance: BEGe, PPC, and ICPC detectors broadly meet the LEGEND energy-resolution goal, whereas the older coaxial detectors have poorer resolution, as already known from GERDA, and are not expected to be relevant for LEGEND-1000, where only ICPC detectors are planned [2509.21166].

In programmatic terms, LEGEND-200 is both a stand-alone search and the technological bridge to the later tonne-scale phase. Earlier design papers defined it as a 200 kg deployment in reused GERDA infrastructure, with MAJORANA electroformed copper, an upgraded liquid-argon veto, improved front-end electronics, and new ICPC detectors as the main ingredients for reaching the $10^{27}$-yr sensitivity class [1905.06572]. The first-year result did not yet realize the ultimate LEGEND-200 background target, but it validated stable low-background operation of enriched HPGe detectors in instrumented liquid argon at substantially increased scale and placed the experiment in direct continuity with the cumulative $^{76}$Ge program [2509.21166].

LEGEND-200 therefore occupies a precise place in the contemporary neutrinoless double beta decay landscape. It inherits GERDA’s bare-detector-in-LAr architecture, MAJORANA’s ultra-clean detector and component development, and the ICPC scaling path established in precursor studies, while producing its own first half-life limit and strengthening the combined $^{76}$Ge constraint. Its technical trajectory remains defined by the same coupled quantities that structured the original roadmap: exposure, energy resolution, signal efficiency, and above all background index.

Source: https://www.emergentmind.com/topics/legend-200