LEGEND-1000: Tonne-Scale 0νββ Search

• LEGEND-1000 is the ton-scale phase of the 76Ge neutrinoless double-beta decay program, deploying 1000 kg of enriched HPGe detectors over a 10 ton·yr exposure.
• It leverages advanced background rejection techniques—including active LAr veto, PSD, and custom ASIC electronics—to achieve a quasi-background-free measurement.
• The experiment targets a discovery sensitivity with a half-life beyond 10^28 years, corresponding to an effective Majorana mass range of approximately 9–21 meV, probing the inverted neutrino mass ordering.

LEGEND-1000 is the planned ton-scale phase of the Large Enriched Germanium Experiment for Neutrinoless Double-Beta Decay, designed to search for $0\nu\beta\beta$ in $^{76}$Ge with $1000$ kg of enriched HPGe detectors, a live time of about $10$ years, and a half-life discovery sensitivity beyond $10^{28}$ years. In the preconceptual design report, the experiment is described as a quasi-background-free search with a 99.7%-CL discovery sensitivity of $1.3\times 10^{28}$ years, corresponding to an effective Majorana mass range of $9$–$21$ meV and intended to cover the inverted-ordering neutrino mass scale (Collaboration et al., 2021).

1. Programmatic position within the LEGEND collaboration

LEGEND is a phased $^{76}$Ge program built from the technical lineages of GERDA and the MAJORANA DEMONSTRATOR. A later collaboration summary states that the collaboration was formed in 2016 to combine GERDA’s operation of germanium detectors in liquid argon with an active LAr veto and MAJORANA’s ultra-clean materials, low-noise electronics, and outstanding energy resolution (Romo-Luque, 26 Aug 2025). Early LEGEND design papers already defined the strategic structure: a $200$ kg stage using existing GERDA infrastructure at LNGS, followed by a tonne-scale stage with discovery potential beyond $^{76}$0 years (Myslik, 2018, Collaboration et al., 2017).

Within that architecture, LEGEND-200 is the intermediate operational phase and LEGEND-1000 is the final large-scale instrument. LEGEND-200 was framed as a $^{76}$1 kg experiment with a target exposure of $^{76}$2 ton·yr and a design background index of $^{76}$3, while LEGEND-1000 is the next-generation phase with $^{76}$4 kg of enriched HPGe and a target exposure of $^{76}$5 ton·yr (Saleh, 25 Sep 2025). This phased structure is not merely administrative. It is the mechanism by which detector geometry, radiopurity protocols, veto instrumentation, electronics, and analysis workflows are validated at the $^{76}$6 kg scale before deployment at the $^{76}$7 kg scale (Romo-Luque, 26 Aug 2025).

A plausible implication is that LEGEND-1000 should be read less as an isolated future detector than as the culmination of an explicitly staged germanium program whose predecessor phase is already producing operational and background-model inputs for the tonne-scale design.

2. Physics rationale and neutrino-mass reach

The experimental target is neutrinoless double-beta decay of $^{76}$8Ge. The process would violate total lepton number by two units, and its observation would imply that neutrinos are Majorana particles; multiple LEGEND summaries also connect this to leptogenesis and the cosmological matter-antimatter asymmetry (Romo-Luque, 26 Aug 2025, Brugnera, 17 Jan 2025). For $^{76}$9Ge, the search is centered on the monoenergetic endpoint signature at $1000$0 (Saleh, 25 Sep 2025).

For the standard light-Majorana exchange mechanism, the decay rate is written as

$1000$1

with

$1000$2

Here $1000$3 is the phase-space factor, $1000$4 the nuclear matrix element, and $1000$5 the effective Majorana mass (Romo-Luque, 26 Aug 2025, Collaboration et al., 2017).

The central physics claim attached to LEGEND-1000 is that sensitivity beyond $1000$6 years in $1000$7Ge reaches the inverted-ordering mass scale. Later LEGEND descriptions state a target $1000$8 reach of approximately $1000$9–$10$0 meV for $10$1 beyond $10$2 years (Saleh, 25 Sep 2025, Saleh, 13 Mar 2026). The preconceptual design report quotes $10$3–$10$4 meV at $10$5 years (Collaboration et al., 2021). Across the design literature, this reach is consistently described as covering the inverted neutrino mass hierarchy or inverted-ordering region (Romo-Luque, 26 Aug 2025, Collaboration et al., 2021).

The sensitivity logic is likewise standard. Large exposure $10$6, very low background index, and HPGe-class energy resolution jointly determine discovery reach. In a background-free or quasi-background-free regime, sensitivity scales approximately linearly with exposure; in a background-limited regime it scales more slowly, roughly with $10$7 (Collaboration et al., 2017, Saleh, 13 Mar 2026). LEGEND-1000 is explicitly optimized to operate in the former regime (Romo-Luque, 26 Aug 2025).

3. Experimental architecture and performance targets

Later LEGEND-1000 descriptions give the core target parameters in a compact form: $10$8 kg of germanium, $10$9 ton·yr exposure, a target background index of $10^{28}$0, and a $10^{28}$1 discovery sensitivity for a $10^{28}$2 half-life beyond $10^{28}$3 (Saleh, 25 Sep 2025). A 2026 LEGEND overview expresses the same background goal as

$10^{28}$4

corresponding to

$10^{28}$5

for $10^{28}$6 keV (Saleh, 13 Mar 2026).

The detector technology baseline has also narrowed with time. Conference and summary papers state that only ICPC detectors are planned for LEGEND-1000 deployment (Saleh, 25 Sep 2025). The rationale is direct: ICPC geometry combines large detector mass with the point-contact field configuration needed for strong PSD and high energy resolution, while avoiding the poorer $10^{28}$7-scale resolution associated with legacy coaxial detectors (Saleh, 25 Sep 2025). The preconceptual design report further specifies germanium detectors enriched to more than $10^{28}$8 in $10^{28}$9Ge, operated in a liquid argon active shield at a deep underground laboratory (Collaboration et al., 2021).

The cryogenic concept is built around nested shielding. The detectors are immersed in LAr, which supplies cooling, passive shielding, and scintillation-based active veto capability. The surrounding infrastructure includes low-radioactivity cryostat materials, external shielding, and muon-veto instrumentation; design studies discuss underground argon in the inner detector region and atmospheric argon in an outer region, preserving an active veto where radiopurity is most critical (Collaboration et al., 2021, Romo-Luque, 26 Aug 2025).

A design-level implication follows from the numerical target itself. At

$1.3\times 10^{28}$0

$1.3\times 10^{28}$1 keV, and $1.3\times 10^{28}$2 kg·yr, the expected background in one FWHM around $1.3\times 10^{28}$3 is well below one count (Saleh, 13 Mar 2026). That quasi-background-free condition is the basis for the claim that LEGEND-1000 could make an unambiguous discovery with only a few counts at the decay $1.3\times 10^{28}$4 value (Collaboration et al., 2021).

4. Background budget and enabling technologies

LEGEND-1000 inherits a layered background-rejection strategy from GERDA, MAJORANA, and LEGEND-200. The relevant subsystems include Ge-Ge anti-coincidence, active LAr scintillation veto, water Cherenkov muon veto, and waveform-based PSD using parameters such as $1.3\times 10^{28}$5 and late-charge observables (Romo-Luque, 26 Aug 2025). Later ASIC studies emphasize that front-end electronics located close to the detector are simultaneously a noise problem and a radiopurity problem; for that reason, dedicated ASIC development has been pursued for LEGEND-1000. Tests of the XGLab CUBE preamplifier with a PPC detector obtained $1.3\times 10^{28}$6 keV FWHM at $1.3\times 10^{28}$7 keV and $1.3\times 10^{28}$8 keV at $1.3\times 10^{28}$9 MeV, with PSD performance comparable to GERDA and MAJORANA (Edzards et al., 2019).

One major background axis is the $9$0Ar/$9$1K chain in LAr. A dedicated LEGEND mitigation study states that $9$2K backgrounds are expected to comprise $9$3–$9$4 of LEGEND backgrounds in both phases if left uncontrolled (Mirza, 2022). The same work studies encapsulation of each Ge detector with ultra-pure plastic, especially PEN, as a scalable mitigation. Simulation results show that about $9$5 mm of PEN would fully absorb a $9$6 MeV electron, while $9$7 mm PEN already produces substantial energy sharing between PEN and Ge, making veto-assisted rejection plausible when combined with dead layers and PSD (Mirza, 2022). This suggests that detector encapsulation is a contingency or complementary mitigation path if underground-sourced argon is limited.

A second major axis is cosmogenic neutrons and the production of $9$8Ge/$9$9Ge. A dedicated comparison of Geant4 and MCNP for LEGEND-1000-like geometries finds that Geant4 generally predicts higher neutron scattering and higher capture rates on germanium than MCNP, implying that earlier Geant4-based cosmogenic estimates are conservative (Barton et al., 2024). The same study proposes methane-doped outer liquid argon as a risk-mitigation option: $21$0 molar methane reduces neutron capture on $21$1Ge by about a factor of $21$2, and $21$3 methane by about a factor of $21$4, without significant modification to the baseline design (Barton et al., 2024).

Two additional R&D lines address event-classification systematics at the waveform level. The CAGE internal-source scanning cryostat was built to study surface events on HPGe detectors; its commissioning with $21$5Am achieved a beam spot precision of $21$6 mm and identified $21$7 as a robust early-rise metric for shallow gamma interactions near passivated surfaces (Othman et al., 6 Feb 2026). Separately, a semi-supervised Affinity-Propagation-plus-SVM cleaning model for LEGEND waveforms reports a maximum sacrifice of physics events of $21$8, and is already being used to accelerate LEGEND-200 data-cleaning development while being positioned for LEGEND-1000-scale analysis workflows (León et al., 2024).

5. LEGEND-200 as the empirical precursor

The first year of LEGEND-200 provides the direct empirical basis for LEGEND-1000 projections. LEGEND-200 began stable physics running in 2023 at LNGS with $21$9 kg of detectors installed, and its first $^{76}$0 analysis used a total exposure of $^{76}$1 kg·yr (Saleh, 13 Mar 2026, Saleh, 25 Sep 2025). Detector types included BEGe, PPC, ICPC, and legacy coaxial units, with enrichment between $^{76}$2 and $^{76}$3 in $^{76}$4Ge (Saleh, 13 Mar 2026).

Performance results are central. ICPC, BEGe, and PPC detectors met the LEGEND-200 energy-resolution goal of $^{76}$5 keV FWHM at $^{76}$6, while coaxial detectors retained the poorer resolution already known from GERDA (Saleh, 13 Mar 2026). After cuts, the first-year background indices were

$^{76}$7

for the golden dataset and

$^{76}$8

for the silver dataset (Saleh, 25 Sep 2025). The frequentist LEGEND-200-only half-life limit was

$^{76}$9

and the combined GERDA + MAJORANA Demonstrator + LEGEND-200 result gave

$200$0

with median exclusion sensitivity

$200$1

described as the best achieved among $200$2 decay searches to date (Romo-Luque, 26 Aug 2025, Saleh, 25 Sep 2025).

For LEGEND-1000, the design consequences are explicit. Coaxial detectors are identified as a dominant source of elevated background and are not planned for future phases; only ICPC are planned for LEGEND-1000 deployment (Romo-Luque, 26 Aug 2025, Saleh, 25 Sep 2025). Background characterization after the first year also triggered screening, cleaning, and redeployment steps in LEGEND-200, including movement toward ICPC-and-BEGe-only arrays and improved surface treatments (Saleh, 13 Mar 2026). This suggests that LEGEND-1000 will not simply scale the first deployed LEGEND-200 hardware mix, but the refined post-characterization configuration.

6. Materials, infrastructure, and role in the global $200$3 program

The LEGEND-1000 program depends on industrially credible enriched-germanium production. A dedicated process-development paper reports hydrogen reduction of $200$4Ge-enriched GeO$200$5 with an average yield of $200$6, subsequent zone refining to intrinsic purity with an overall Ge yield of $200$7, and an average cosmogenic exposure of $200$8 h for a $200$9 kg enriched batch (Gradwohl et al., 2020). The authors state that the line was developed to find the optimum solution for processing large quantities of germanium for LEGEND-1000 (Gradwohl et al., 2020). This suggests that isotope logistics and feedstock preparation, not only detector operation, are recognized as core enabling technologies for the tonne-scale phase.

Mechanical infrastructure is likewise under active validation. A recent LEGEND-1000 hardware campaign on OFHC copper cylinders for the reentrant-tube design used hydrostatic burst tests and an automated vision pipeline to reconstruct hoop stress-strain curves and extract yield strengths by the $^{76}$00 offset, $^{76}$01 EUL, and Johnson-Cook methods; the same work cross-validated video-derived diameter extraction against a non-agentic pipeline at the $^{76}$02 pixel level and compared results to Ansys simulations (Simonaitis-Boyd et al., 15 Jun 2026). The presence of such studies indicates that LEGEND-1000 design work extends beyond detector modules into long-duration cryogenic vessel mechanics and dual-argon-volume containment.

In the broader $^{76}$03 landscape, LEGEND-1000 is the flagship germanium-based next-generation experiment. Later LEGEND summaries emphasize the combination of source-equals-detector geometry, $^{76}$04 energy resolution at $^{76}$05, powerful PSD, and multi-layer active vetoes as the distinctive strengths of the $^{76}$06Ge approach (Saleh, 25 Sep 2025, III et al., 2019). The same literature places LEGEND-1000 among the primary experiments capable of decisively testing the inverted-ordering parameter space for Majorana neutrinos (Collaboration et al., 2021, Myslik, 2018).

If LEGEND-1000 observes $^{76}$07, the implication would be lepton-number violation and Majorana neutrinos. If it reaches its intended sensitivity and observes no signal, the standard light-Majorana interpretation of the inverted-ordering region would be strongly disfavored (Romo-Luque, 26 Aug 2025, Collaboration et al., 2021). In either outcome, LEGEND-1000 is designed as a decisive $^{76}$08Ge experiment: not merely a scale-up of earlier germanium arrays, but a tonne-scale, quasi-background-free instrument built to turn sub-$^{76}$09 meV neutrino-mass physics into an experimental question.