The Quest for No Neutrinos: �Advancing the Search with LEGEND -1000

Moritz Neuberger (TUM) - ENTENTE, SNOLAB - 30 Sep 2025

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On behalf of the LEGEND collaboration

Search for 0𝜈ββ in ⁷⁶Ge enriched HPGe

High Purity Germanium (HPGe) detectors enriched in ⁷⁶Ge

• source = detector → high efficiency
• pure → negligible intrinsic background
• Ge crystal → very good energy resolution (0.1% @ Q_ββ)
• pulse-shape discrimination

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Q_ββ = 2039 keV

ββ

e^-

h

Background is approximately flat near Q_ββ. �

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with E the exposure, DeltaE the resolution, �and BI the background index in .

Expected signal and background signature

[10.48550/arXiv.2107.11462]

Moritz Neuberger (TUM) - ENTENTE, SNOLAB - 30 Sep 2025 - The Quest for No Neutrinos: Advancing the Search with LEGEND -1000

The History of ⁷⁶Ge based 0𝜈ββ Search

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GERDA�(127.3 kg yr)

Had the best BI:

5 × 10^-4 cts/(keV kg yr)

Used both PSD and LAr anti-coincidence.

Majorana Demonstrator

(64.5 kg yr)

Had the best energy resolution:

2.5 keV @ Q_ββ

Used PSD to reduce BI.

Moritz Neuberger (TUM) - ENTENTE, SNOLAB - 30 Sep 2025 - The Quest for No Neutrinos: Advancing the Search with LEGEND -1000

The Future of ⁷⁶Ge based 0𝜈ββ Search

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10^-5 cts/(keV kg yr)

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2 × 10^-4 cts/(keV kg yr)

5 × 10^-4 cts/(keV kg yr)

LEGEND-200

LEGEND-1000

Reminder:

0.05x

10x

Moritz Neuberger (TUM) - ENTENTE, SNOLAB - 30 Sep 2025 - The Quest for No Neutrinos: Advancing the Search with LEGEND -1000

The Future of ⁷⁶Ge based 0𝜈ββ Search

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10^-5 cts/(keV kg yr)

​

2 × 10^-4 cts/(keV kg yr)

5 × 10^-4 cts/(keV kg yr)

LEGEND-200

LEGEND-1000

Reminder:

0.05x

10x

[9-24] meV

@T_1/2 = 10²⁸yr

Moritz Neuberger (TUM) - ENTENTE, SNOLAB - 30 Sep 2025 - The Quest for No Neutrinos: Advancing the Search with LEGEND -1000

LEGEND -200: Setup and Status

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142 kg of 86%-92% enriched HPGe installed

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fiber “curtain”

SiPM

SiPM

@LNGS Hall A

liquid argon cryostat

instrumented water tank for muon veto

LAr instrumented around HPGe �

Last operation (2023-2024):

More information on this in Andreas Leonhardt’s talk on Friday!

Moritz Neuberger (TUM) - ENTENTE, SNOLAB - 30 Sep 2025 - The Quest for No Neutrinos: Advancing the Search with LEGEND -1000

LEGEND -1000 @ LNGS Hall C

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atmospheric argon

neutron moderator

underground argon�in re-entrant vessle

string modules

IC detectors

1000 kg of IC HPGe detectors

Moritz Neuberger (TUM) - ENTENTE, SNOLAB - 30 Sep 2025 - The Quest for No Neutrinos: Advancing the Search with LEGEND -1000

Background Suppression Techniques in LEGEND

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Recent LEGEND-200 results:

[10.1103/25tk-nctn]

a.k.a. standard cuts:

Moritz Neuberger (TUM) - ENTENTE, SNOLAB - 30 Sep 2025 - The Quest for No Neutrinos: Advancing the Search with LEGEND -1000

Background Budget for LEGEND-1000

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L200

L1000

L1000 Background Goal

×0.05

L200

Projected energy distribution for LEGEND-1000:

Projected individual background contributions:

Moritz Neuberger (TUM) - ENTENTE, SNOLAB - 30 Sep 2025 - The Quest for No Neutrinos: Advancing the Search with LEGEND -1000

Background Budget for LEGEND-1000

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Different Monte-Carlo realizations

We need a low background to achieve an �unambiguous discovery of 0νββ decay with just a handful of counts.

radiogenic

Moritz Neuberger (TUM) - ENTENTE, SNOLAB - 30 Sep 2025 - The Quest for No Neutrinos: Advancing the Search with LEGEND -1000

Radiogenic Background - Clean and active material

Clean materials:

• Thorough screening to identify final materials �(gamma counting, radon emanation and surface screening)
• In-house production for high-risks parts: (i.e. close to detectors)
• scintillating PEN base-plates
• electroformed (EF) copper

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More active material: (i.e. sensitive, either HPGe bulk or optically)

• larger inverted coaxial (IC) detectors result in larger ratio of bulk (active) to surface (inactive)
• less inactive structural material improves LAr rejection

The total expected radiogenic background is:

(2.2±0.8) × 10^-6 cts/(keV kg yr) �or ~22% of the total budget

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PEN base-plate

EF copper mounting structure

BEGe detectors�usually ~0.5 kg

IC detector

usually ~3 kg

Picture from LEGEND-200

Moritz Neuberger (TUM) - ENTENTE, SNOLAB - 30 Sep 2025 - The Quest for No Neutrinos: Advancing the Search with LEGEND -1000

⁴²K background - UnderGround Liquid Argon (UGLAr)

⁴²K is a daughter of the cosmogenic ⁴²Ar (T_1/2 = 33 yr). �After its production it can drift to the n⁺ surface of HPGes. βs can pass into the bulk and mimic 0𝜈ββ decay.

AtmLAr: 0.72 cts/(keV kg yr) (before cuts)

UGLAr: reduction of ⁴²Ar by factor <7 × 10^-4� + strong PSD suppression of ⁴²K decays�→ total survival fraction: SF = 5.5 × 10^-6

The estimated ⁴²K background is:

(4.0^+2.2 ) × 10^-6 cts/(keV kg yr) �or ~40% of the total budget (largest single contributor)

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Re-entrant vessel filled with UGLAr:

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⁴²Ar (uniform)

β of ⁴²K decay likely enters HPGe

[10.48550/arXiv.2107.11462]

The cosmogenic ⁴²Ar is present in commercial argon. �(Atmospheric LAr ~ 40 - 110 mBq/t) [1,2].

After production, ⁴²K is charged, drifts towards HPGe and decays close to HPGe (0.73 cts/(keV kg yr) before cuts) [3].

GERDA/LEGEND-200 solution:

physical barrier to reduce collection volume �(however, suppression not sufficient to reach LEGEND-1000 background goal).

LEGEND-1000 solution:

UGLAr from wells depleted in ⁴²Ar (<7×10^-4) [3] , 22 ton

+3000V

Combined with PSD suppression due to late charges,

the ⁴²K decay survival fraction (SF) is 5.5 x 10^-6

[1] GERDA internal report (2016)

[2] DEAP Collaboration, arXiv:1905.05811 (2019)

[3] N. Abgrall et al. (LEGEND Collaboration), arXiv:2107.11462 (2021)

Moritz Neuberger (TUM) - ENTENTE, SNOLAB - 30 Sep 2025 - The Quest for No Neutrinos: Advancing the Search with LEGEND -1000

⁴²K background - alternative

Optically Active HPGe detector enclosures e.g. PEN.

LAr test stand doped with ⁴²Ar at found, �that with PEN enclosures and background cuts �a SF < 1.6× 10^-4 (90% CL) is possible [1].

Future prospects:

• more ⁴²Ar to improve sensitivity
• optimized geometry for enclosures
• specialized PSD based on machine learning

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Resulting energy spectra:

In the test stand:

PEN Enclosures

[1] Christoph Vogl et. al., ⁴²K mitigation for LEGEND, TAUP 2025

Moritz Neuberger (TUM) - ENTENTE, SNOLAB - 30 Sep 2025 - The Quest for No Neutrinos: Advancing the Search with LEGEND -1000

Delayed µ-induced background - ^77mGe dominates

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In-situ muon-induced production of isotopes

Decay background contribution (after standard cuts)

De-excitation from compound after ⁷⁶Ge(n,γ):

Moritz Neuberger (TUM) - ENTENTE, SNOLAB - 30 Sep 2025 - The Quest for No Neutrinos: Advancing the Search with LEGEND -1000

Delayed µ-induced background - Production Cut (for ^77mGe)

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Two tagging strategies:

1. Direct approach identifying the ⁷⁶Ge(n,γ) cascades
2. Indirect approach identifying captures of sibling neutrons

Strategy:

Identify the production of ^77mGe using its production topology, then reject 0nββ candidates up to 5τ (~6.5 min) after.

Moritz Neuberger (TUM) - ENTENTE, SNOLAB - 30 Sep 2025 - The Quest for No Neutrinos: Advancing the Search with LEGEND -1000

Delayed µ-induced background - Production Cut (for ^77mGe)

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In most ⁷⁶Ge(n,γ) cascades (~80%), an energy of �>250 keV is deposited in the containing detector.

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If >250 keV is deposited up to 3ms after a muon, tag this HPGe as ^77mGe-producing.

1. Direct tagging:

Energy deposited in HPGe in ^77mGe production / all muons

Moritz Neuberger (TUM) - ENTENTE, SNOLAB - 30 Sep 2025 - The Quest for No Neutrinos: Advancing the Search with LEGEND -1000

Delayed µ-induced background - Production Cut (for ^77mGe)

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2. Indirect tagging:

Production correlates with hadronic muon showers with many neutrons.

Count γ-cascades via p.e. thresholds and consider time profile.

Tag µ as ^77mGe-producing when the number of γ-cascades in the event exceeds a threshold.

Location:

Timing after µ:

Correlation of # detected γ-cascades per muon�in sensitive volumes in ^77mGe production:

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Sensitive volumes

Deposited Energy per response window (5µs):

Moritz Neuberger (TUM) - ENTENTE, SNOLAB - 30 Sep 2025 - The Quest for No Neutrinos: Advancing the Search with LEGEND -1000

Side note: neutron moderator and ALAr instrumentation

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Energy deposition of ⁴⁰Ar(n,γ) [1]:

Light detection probability [1]:

Neutron moderator:

• 12 panels made of PMMA (3m x 1m x 10cm)
• reduces ^77mGe-production by factor 56%

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Light readout of ALAr via light guids into SiPM:

• most sibling neutrons are captured in the LAr around the moderator
• optical readout close to where most light emitted after ⁴⁰Ar(n,γ)

[1] Michele Morella, PhD thesis, GSSI (2025)

[1]

Moritz Neuberger (TUM) - ENTENTE, SNOLAB - 30 Sep 2025 - The Quest for No Neutrinos: Advancing the Search with LEGEND -1000

Delayed µ-induced background - Production Cut (for ^77mGe)

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Tagging conditions based on calorimetric simulations found by optimising the discovery sensitivity:

If a HPGe detector / µ is tagged as ^77mGe-producing, reject 0nββ candidates up to 5τ (~6.5 min) after.

Moritz Neuberger (TUM) - ENTENTE, SNOLAB - 30 Sep 2025 - The Quest for No Neutrinos: Advancing the Search with LEGEND -1000

Delayed µ-induced background - De-excitation Cut (for ⁷⁷Ge)

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Strategy:

Tag ⁷⁷Ge decays via the delayed de-excitation �from ^77mAs (475 keV, 114 µs).

Implementation:

Cut 0νββ candidates if the same detector has >200 keV deposited within 1 ms after.

Moritz Neuberger (TUM) - ENTENTE, SNOLAB - 30 Sep 2025 - The Quest for No Neutrinos: Advancing the Search with LEGEND -1000

Delayed µ-induced background - Performance

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With the production and de-excitation cut, the total delayed µ-induced background is

with only a marginal reduction in 0𝜈ββ decay survival fraction.

The total background is estimated at

Decay background contribution:

with which the physics goal is reachable�i.e. discovery sens. of >10²⁸ yr (99.73% CL).

(9.2^+5.0 ) × 10^-6 cts/(keV kg yr)

_-6.5

Moritz Neuberger (TUM) - ENTENTE, SNOLAB - 30 Sep 2025 - The Quest for No Neutrinos: Advancing the Search with LEGEND -1000

Summary

Preparation for LEGEND -1000 at LNGS ongoing and construction starts 2026.

To reach the background goal:

• radiogenic: clean and active material
• ⁴²K: UGLAr, active enclosures as alternative
• µ-induced: topology based tagging to reject decays of in-situ produced isotopes

With this, we can make an unambiguous discovery of 0νββ decay with just a handful of counts at Q_ββ.

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Design and Physics Goals

Moritz Neuberger (TUM) - ENTENTE, SNOLAB - 30 Sep 2025 - The Quest for No Neutrinos: Advancing the Search with LEGEND -1000

Backup

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Reminder: Neutrinoless Double Beta Decay

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Neutrinoless double beta decay (0𝜈ββ)

• a beyond SM process that violates lepton-number conservation (ΔL=2)
• has not been observed so far, but
• its detection would confirm the Majorana character of neutrinos.

Double beta decay (2𝜈ββ)

• an observed standard model (SM) process

Moritz Neuberger (TUM) - ENTENTE, SNOLAB - 30 Sep 2025 - The Quest for No Neutrinos: Advancing the Search with LEGEND -1000

Reminder: Link between 0𝜈ββ and 𝜈-mass

Assuming light Majorana neutrino exchange

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with

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Next generation 0𝜈ββ experiments aim to probe the entire inverted mass ordering region

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graph by Christoph Wiesinger

half life - ββ-mass relation

ββ-mass (coherent sum)

Moritz Neuberger (TUM) - ENTENTE, SNOLAB - 30 Sep 2025 - The Quest for No Neutrinos: Advancing the Search with LEGEND -1000

Comparison to other experiments

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Moritz Neuberger (TUM) - ENTENTE, SNOLAB - 30 Sep 2025 - The Quest for No Neutrinos: Advancing the Search with LEGEND -1000

Nuclear matrix element and phase space

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Moritz Neuberger (TUM) - ENTENTE, SNOLAB - 30 Sep 2025 - The Quest for No Neutrinos: Advancing the Search with LEGEND -1000