LEGEND: 0νββ decay search with germanium detectors

LEGEND: 0νββ decay search with germanium detectors Konstantin Gusev on behalf of LEGEND collaboration 30.09.2025 0νββ search: why? 2νββ Possible in 35 even-even nuclei (βdecay is energy/spin suppressed) •Rare process with half life is 1010 longer than the age of the universe, however already observed in 14 isotopes! •Most precise measurement of 2νββ half-life in the world by GERDA: 0νββ search: why? 2νββ 0νββ •Violates lepton number •Forbidden in Standard Model •New BSM physics •Creates matter w/o antimatter •Shows, that ν has Majorana mass component •In case of light ν exchange •would give access to ν mass scale •would provide important input to cosmology •Rare process with half life is 1010 longer than the age of the universe, however already observed in 14 isotopes! •Most precise measurement of 2νββ half-life in the world by GERDA: Possible in 35 even-even nuclei (βdecay is energy/spin suppressed) Link between 0νββ and 𝜈𝜈-mass 1 𝑇𝑇1/2 0𝜈𝜈 =𝐺𝐺0𝜈𝜈(𝑄𝑄,𝑍𝑍)𝑔𝑔𝐴𝐴 4𝑀𝑀0𝜈𝜈 2 𝑚𝑚𝛽𝛽𝛽𝛽 𝑚𝑚𝑒𝑒 2 Rev. Mod. Phys. 95, 025002 (2023) Inverted Ordering Normal Ordering 0νββ decay rate Phase space factor Nuclear Matrix Element Effective Majorana neutrino mass Link between 0νββ and 𝜈𝜈-mass Notes: •Recent KamLAND-Z 800 limit came to IO region •Next generation projects fully cover IO and part of NO 1 𝑇𝑇1/2 0𝜈𝜈 =𝐺𝐺0𝜈𝜈(𝑄𝑄,𝑍𝑍)𝑔𝑔𝐴𝐴 4𝑀𝑀0𝜈𝜈 2 𝑚𝑚𝛽𝛽𝛽𝛽 𝑚𝑚𝑒𝑒 2 Rev. Mod. Phys. 95, 025002 (2023) Inverted Ordering Normal Ordering KLZ-800 (2023), largest NME Next generation goal Experimental design considerations 𝑇𝑇1/2 0𝜈𝜈 ∝ 𝜖𝜖 𝑀𝑀 𝑡𝑡 ∆𝐸𝐸 𝐵𝐵𝐵𝐵 •Detector performance: –High mass and good long-term stability to increase exposure –High efficiency: source = detectors –Good energy resolution –Small background: •Underground labs to reduce the cosmogenic •Materials handling and cleanliness •Strict radiopurity constraints •Passive and active (!) shielding •Signal discrimination techniques Background Background Index “Background free” experiment •But energy resolution still essential! 𝑇𝑇1/2 0𝜈𝜈 ∝ 𝜖𝜖 𝑀𝑀 𝑡𝑡 Experimental landscape 76Ge 136Xe 130Te 100Mo 82Se Rev. Mod. Phys. 95, 025002 (2023) ICPC n+ contact (lithiated) @ 3 - 5 kV ββ p+ contact (boron implanted) 104kg yr exposure 2.5 keV FWHM 0νββ search with HPGe detectors enriched in 76Ge 2.043 •High-Purity Germanium detectors enriched in 76Ge –ββ source = detector → high efficiency –high purity → low intrinsic background –isotope enrichment → ≳90 % 76Ge –excellent energy resolution → ~ 0.1 % FWHM @ Qββ –topological discrimination → pulse shape discrimination (PSD) Wavelength shifting reflector HPGe Detector array & LAr Instrumentation Water tank / μ-Veto liquid Ar cryostat HPGe readout electronics based on MJD Low Mass Front-End and GERDA charge sensitive amplifier (CC4) Detector mount: underground copper, optically active PEN plates &radiopure plastics Larger mass (inverted coaxial) HPGe detectors with up to 4kg Source funnels for 228Th calibration sources Liquid Argon instrumentation: inner &outer fiber barrels with silicon photomultiplier (SiPM) readout at top &bottom LEGEND-200: the best from GERDA and Majorana LEGEND-200: Taking first data 142 kg installation: •Installation of all available HPGe detectors (101 detectors in 10 strings) as well as full LAr installation, DAQ, electronics (Oct’22) •130 kg operational (12 kg off due to hardware issues) •LAr instrumentation operational •About 1 year of data taking •First results with 48 kg⋅yr exposure, updated with 61 kg⋅yr Maintenance work started in 2024 and finished recently 8% 10% 16%66% LEGEND-200: Energy Scale and Resolution •Energy scale evaluated by weekly 228Th calibration between physics runs •Most detectors fulfill LEGEND energy resolution goal (0.12% at Qββ) •Stable energy scale among calibrations •Data partitioned according to stability of energy observables LEGEND-200: Signal and background discrimination Blinding applied at Qββ = 2039 keV (50 keV window) •Data cleaning: 95-99% survival after removal of unphysical events •Muon veto: 2 events removed at Qββ •Multiplicity cut: 26% of events rejected near Qββ blinded energy range LEGEND-200: Quality, Muon Veto and Multiplicity Bayesian background model using data before analysis cuts •fitsreproduced data well •estimates 2-3x higher 208Tl / 214Bi compared to radio assay expectation •comprehensive screening campaign performed •cleaning procedures re-evaluated for the new deployment fit: radio assay expectation: (no fit) LEGEND-200: Modeling before Analysis Cuts ●Pulse shape classifier: A/E =max(current) /energy bulk single site bulk multi site surface βand αevents (0𝜈𝜈ββ) AE LEGEND-200: HPGe Pulse Shape Discrimination ●Strong suppression of surface α and β (42K) events ●~60% suppression of Compton multi-site events at Qββ ●0νββ decay survival fraction of ~85% LEGEND-200: Spectrum after PSD (e.c.) (β-) ●Strong suppression of background above 2𝜈𝜈ββ ●ββ decay signal acceptance of ∼93% Signal w/o scintillation light Background w/ scintillation light LEGEND-200: Argon anti-coincidence LEGEND-200: Spectrum after all cats ●PSD and argon cuts are complementary ●232Th strongly suppressed and remaining Compton background vanishes ●“Pure” 2νββ distribution at lower energies, 11 events surviving at Qββ: 9 before + 2 after unblinding http://arxiv.org/abs/2505.10440 LEGEND-1000: Background Model Simulated total background spectrum after cuts Expected background contributions (preliminary) Color bands correspond to 1σ uncertainty or 90%UL Projected background index ~ 10−5 cts/(keV kg yr) ●LEGEND will span the inverted ordering and a large part of the normal ordering space ● Discovery sensitivity <18.4 meV for 12/15 calculations Agostini, Benato, Detwiler, Menéndez and Vissani PRC 104, L042501 (2023 Update) 33 LEGEND-1000:Sensitivity 0νββ T1/2 =1028 yr 3-4 events Flat, featureless background No background peaks are expected close to Qββ 2νββ <10-6 2νββ events leak into in Qββ±2σ Simulated example spectrum: after cuts from 10 years of data ≈0.1%FWHM energy resolution LEGEND-1000: Designed for an Unambiguous Discovery LEGEND-1000: Timeline &Outlook •LEGEND-1000 is optimized for aquasi-background-free 0νββ search –It builds on breakthrough developments by GERDA, MAJORANA,and LEGEND-200 –LEGEND has alow-risk path to meeting its background goal of 10-5 counts/(keV kg yr) –Low backgrounds, excellent resolution, and event topology discrimination allow for an unambiguous discovery of 0νββ decay at T1/2 =1028 years LEGEND Website https://legend-exp.org/ 2032 2033 2034 Construction, Detector Production &Installation 2035 2036 Full Data Taking 2023 2024 2025 2026 2027 2028 2029 2030 2031 Design &Reviews First Data *Technically driven schedule Summary •LEGEND-200 –collected over a year of exposure –first unblinding with combined world-leading sensitivity: –background studies completed –array redeployed in Summer 2025 with newly produced detectors –data taking restarted •LEGEND-1000 –design is well underway –technical preparations @ LNGS started –funding is being pursued in the US and Europe 2.8 × 1026 yr Summary •LEGEND-200 –collected over a year of exposure –first unblinding with combined world-leading sensitivity: –background studies completed –array redeployed in Summer 2025 with newly produced detectors –data taking restarted “…an era in which a discovery could come at any time!” •LEGEND-1000 –design is well underway –technical preparations @ LNGS started –funding is being pursued in the US and Europe 2.8 × 1026 yr Backup slides 208Tl DEP 212Bi FEP 208Tl SEP 208Tl FEP 39 208Tl SEP 208Tl FEP 208Tl DEP 212Bi FEP Th calibration ●Need to accept SSE (e.g. ββ) ●Reject the rest ●Optimize with calibration Single site event (e.g. 208Tl DEP, ββ) Multi site event (e.g. 212Bi FEP) p+surface event n+surface event (e.g. incomplete charge collection) PSD in details 2 Ar* Ar Ar Excitation Ionization VUV scintillation light (128 nm) TPB visible blue light (450 nm) Shifting to green light (500 nm) Fibers are vacuum evaporated with TPB Fibers SiPM (100 µm pitch) Signal from SiPMs Signal Ge detector anticoincidence ? ? LAr scintillation principle It restricts the LAr volume around the detectors. Also shifts scintillation light to blue and reflects it back towards the LAr instrumentation Wavelength Shifting Reflector