Results of CUORE A search for 0 νββ of 130 Te Stefano Dell’Oro, on behalf of the CUORE Collaboration Center for Neutrino Physics, CUORE Virginia Polytechnic Institute and State University Rencontres de Moriond – Electroweak Interactions and Unified Theories March 16 – 23, 2019 - La Thuile (AO), Italy
Neutrinoless double beta decay & thermal detectors CUORE A powerful search has to aim at the optimal isotope + detector technique combination • 130 Te is an ideal candidate for the 0 νββ search • Q ββ moderately high: (2527 . 515 ± 0 . 013) keV (between the 208 Tl peak and Compton edge) • large natural abundance: (34 . 167 ± 0 . 002)% • Tellurium dioxide, TeO 2 , suitable for the use in cryogenic particle detectors • high Debye temperature: ⇒ small heat capacity • thermal expansion close to copper • production of high-quality crystals • large mass: ∼ 750 g (5 × 5 × 5 cm 3 ) • scalability of detector arrays • very low radioactive contamination • bulk: 10 − 14 g/g for both U and Th • surface: < 10 − 9 Bq cm − 2 for both U and Th CUORE crystal S. Dell’Oro Results of CUORE La Thuile – March 19, 2019 2 / 18
Working principle CUORE Simplified thermal model • bolometers detect the phonon contribution of the energy released G Heat Absorber • large fraction of the total energy bath Phonon • ionization/excitation → · · · → phonons T 0 C sensor • measured via temperature variation • an absorber with heat capacity C • (connected to) a heat bath @ constant T 0 • (through) a thermal conductance G • ∆ T = ∆ E / C • low C : C ↓ ⇒ ∆ T ↑ 1000 • very low T ∆ T ( t ) = ∆ E C e − t 800 τ • Debye law: C ∝ ( T / Θ D ) 3 V [mV] 600 • thermal fluctuations ∝ T 2 C 400 • temporal evolution: τ = C / G 200 0 • NTD Ge thermistor 0 1 2 3 4 5 t [s] • R = R ∗ exp ( T ∗ / T ) 1/2 S. Dell’Oro Results of CUORE La Thuile – March 19, 2019 3 / 18
CUORE at the Laboratori Nazionali del Gran Sasso CUORE CUORE • LNGS → ideal place to search for 0 νββ B A • 3600 m. w. e. overburden C • µ : 3 · 10 − 8 cm − 2 s − 1 / n: 4 · 10 − 6 cm − 2 s − 1 • dedicated facilities to run bolometric detectors LNGS • Hall A dilution refrigerator (1989) • crystals (1991 – 1995) • MiDBD (1998 – 2001) 10 26 CUORE • Cuoricino (2003 – 2008) 10 25 • CUORE-0 (2013 – 2015) CUORE-0 [yr] Cuoricino 10 24 • CUORE cryostat (2016) (90% C. L.) MiDBD 10 23 4 crystal array • CUORE (from 2017) 334 g 10 22 73 g / 2 t 0 ν 10 21 1 34 g 10 20 30-year-long history 21 g Rev. Sci. Instrum. 89, 121502 (2018) 6 g 10 19 of measurements 1990 1995 2000 2005 2010 2015 2020 2025 Running period S. Dell’Oro Results of CUORE La Thuile – March 19, 2019 4 / 18
CUORE detector CUORE CUORE: Cryogenic Underground Observatory for Rare Events • largest bolometric detector ever built by a factor 10 • 19 towers × 13 floors × 4 crystals = 988 bolometers • 1 tonne detector mass: 330 kg Cu + 742 kg TeO 2 → 206 kg of 130 Te • design goals on performance • 5 keV FWHM energy resolution @ 2615 keV • 0.01 counts keV − 1 kg − 1 yr − 1 in the 0 νββ region • primary goal: search for 0 νββ of 130 Te • measurement of 2 νββ half-life + Te rare decays • search for DM candidates (WIMPs, axions, . . . ) • study of the bolometric thermal behavior CUORE requires a dedicated • investigation of background for cryogenic system in order to be operated as a bolometer next generation 0 νββ experiments S. Dell’Oro Results of CUORE La Thuile – March 19, 2019 5 / 18
A 10 mK infrastructure for large bolometric arrays CUORE • the design of the CUORE cryostat had to satisfy very tight requirements • large experimental volume for detector + shielding of ∼ 1 m 3 • base temperature for optimal operation of NTDs, i. e. down to 10 mK • low radioactive background from the cryogenic apparatus, compatible with goal of 0 . 01 counts keV − 1 kg − 1 yr − 1 at Q ββ • high system reliability to guarantee long-term operation • response to seismic events (LNGS are located in a seismic sensitive area) • custom cryogen-free cryostat • only a few construction materials acceptable • use of Cu OFE/Cu NOSV for plates and vessels • more than 6.5 t of lead shielding integrated in the structure S. Dell’Oro Results of CUORE La Thuile – March 19, 2019 6 / 18
Cryostat configuration CUORE • 6+1 thermal stages paper in preparation Dilution • 300 K @ ambient temperature Pulse Unit Tube • 40 K @ PT first stage temperature • 4 K @ PT second stage temperature 300K • Still @ 800 mK Outer 40K • HEX @ 50 mK Vacuum 4K Chamber • MC @ base T < 10 mK Still • TSP @ stabilized working T Inner Heat Vacuum • 2 vacuum chambers EXchan. Chamber Mixing • Fast Cooling System + Top Chamber 5 Pulse Tubes + custom Dilution Unit Lead • 2 internal lead shields Tower CUORE Support • use of ancient Roman lead detectors Plate • Spanish ingots from I century BCE Internal 210 Pb activity < 4 mBq kg − 1 • Lead Shield S. Dell’Oro Results of CUORE La Thuile – March 19, 2019 7 / 18
Some pictures . . . CUORE DU Superinsulation Plates + Top Lead PT Detector/Top Lead suspensions Vessels Inside the IVC S. Dell’Oro Results of CUORE La Thuile – March 19, 2019 8 / 18
CUORE road map CUORE • tower assembly (Sep 2012 – Jul 2014) CUORE cool down • cryostat commissioning (Aug 2012 – Mar 2016) CUORE cool down Start: 2016-12-05 00:00 100 • detector installation (Jul – Aug 2016) T [K] 10 40K plate 4K plate 0 2 4 6 8 10 12 14 16 18 20 22 ∆ t [d] • cool down: T MC = 6.8 mK 10 1 First observed events T [K] Still plate HEX plate 0.1 MC plate CUORE cool down Start: 2017-01-23 10:00 0 . 01 0 0.5 1 1.5 2 2.5 3 3.5 4 ∆ t [d] S. Dell’Oro Results of CUORE La Thuile – March 19, 2019 9 / 18
CUORE first science run CUORE 15.5 • Begin of Physics data-taking (Apr 2017) 15.4 15.3 15.2 • working T set to 15 mK T [K] 15.1 15 • Dataset 1 : 3 weeks of physics data 14.9 14.8 • further optimization campaign 14.7 Start: 2017-05-05 h12:00 14.6 • Dataset 2 : 4 weeks of physics data 0 5 10 15 20 25 30 ∆ t [d] • collected exposure for 86 . 3 kg yr of TeO 2 (24 . 0 kg yr of 130 Te) Operational performance 25% 48% • 99.6% of channels active (984/988) • energy resolution at Q ββ of 7.7 keV FWHM 19% • signal efficiency of ∼ 80% 8% <1% • . . . room for improvement ⇒ maximize sensitivity Background Test • cryogenic stability Calibration Setup • calibration/background ratio Other S. Dell’Oro Results of CUORE La Thuile – March 19, 2019 10 / 18
Results on the search for 0 νββ CUORE • no peak found at Q ββ ROI spectrum • bkg index consistent with expectations: Phys. Rev. Lett. 120, 132501 (2018) (1 . 4 ± 0 . 2) · 10 − 2 counts keV − 1 kg − 1 yr − 1 2 0 -2 • median statistical sensitivity: 16 / 2 = 7 . 0 · 10 24 yr @ 90% C. L. t 0 ν Events/(2.5 _ keV) 14 1 12 10 • combined limit on 130 Te: 8 / 2 > 1 . 5 · 10 25 yr @ 90% C. L. 6 t 0 ν 1 4 2 20 0 18 CUORE 2480 2500 2520 2540 2560 Negative Log Likelihood CUORE-0 16 Reconstructed Energy [keV] Cuoricino 14 Combined 12 10 1 8 100 Mo 82 Se 6 130 Te (CUORE-2018) 76 Ge 4 10 -1 136 Xe 2 IH 0 -0.1 -0.05 0 0.05 0.1 0.15 0.2 0.25 0.3 Decay rate [10 − 24 yr − 1 ] 10 -2 NH 10 -3 • limit on the effective Majorana mass: m ββ > (110 − 520) meV 10 -4 10 -4 10 -3 10 -2 10 -1 10 -4 10 -3 10 -2 10 -1 1 S. Dell’Oro Results of CUORE La Thuile – March 19, 2019 11 / 18
Background spectrum CUORE [counts keV − 1 kg − 1 yr − 1 ] CUORE-0 100 CUORE Event Rate 10 1 0.1 CUORE Preliminary Exposure: 86.3 kg.yr 1000 2000 3000 4000 5000 6000 7000 Reconstructed Energy [keV] • in general, background consistent with expectations • γ s significantly reduced • most α s compatible with CUORE-0 • 210 Po excess likely from shallow contamination in copper around the detectors • still working on it • estimated contribution to ROI at level of 10 − 4 counts keV − 1 kg − 1 yr − 1 S. Dell’Oro Results of CUORE La Thuile – March 19, 2019 12 / 18
CUORE background budget CUORE Eur. Phys. J. C, 77, 543 (2017) TeO 2 : natural radioactivity TeO 2 : cosmogenic activation Cu NOSV : natural radioactivity Cu NOSV : cosmogenic activation Cu OFE : natural radiocativity Pb Roman : natural radioactivity Pb modern : natural radioactivity Super Insulation : natural radioactivity Stainless steel : natural radioactivity Environmental μ Environmental n 90% C.L. limit value Environmental γ 1E-06 1E-05 1E-04 1E-03 1E-02 1E-01 [counts keV -1 kg -1 keV -1 ] cosmogenic activation Te CUORE-0 bkg model material screening environmental fluxes S. Dell’Oro Results of CUORE La Thuile – March 19, 2019 13 / 18
Modeling the background CUORE • simulation of contamination from different cryostat components with Geant4 MC • background sources identified/ascribed to different locations in experimental setup • inputs of MC • coincidence analysis, gamma peaks, alpha peaks • radio-assay measurements, data from neutron activation • splitting data • multiplicities: sensitive to different types of bkg . . . • inner and outer layers: utilize self shielding by the outer layers top side S. Dell’Oro Results of CUORE La Thuile – March 19, 2019 14 / 18
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