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The search for neutrinoless double beta decay with Cuoricino and Cuore

The search for neutrinoless double beta decay with Cuoricino and Cuore. Elena Guardincerri – Laboratori del Gran Sasso On behalf of the CUORE collaboration. Status of v Physics. Neutrino oscillation experiments proved that neutrinos are mixed and massive. Open questions:

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The search for neutrinoless double beta decay with Cuoricino and Cuore

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  1. The search for neutrinoless double beta decay with Cuoricino and Cuore Elena Guardincerri – Laboratori del Gran Sasso On behalf of the CUORE collaboration Discrete Conference - Elena Guardincerri

  2. Status of v Physics Neutrino oscillation experiments proved that neutrinos are mixed and massive Open questions: • Absolute mass scale and the hierarchypattern • Type of fermion: Dirac or a Majorana? • 13 and values of the CP phases Motivations for  Discrete Conference - Elena Guardincerri

  3. 2n - bb decay u - e d e W n e d n W - e u 0n - bb decay Neutrinoless double beta decay • Double beta decay • 2nd order SM weak process • already observed with lifetimes t~ 1019-1021 y • Neutrinoless double beta decay • Not allowed by SM (it violates L) • Claim of observation in 76Ge: t = 2.23 x 1025 y • Mod.Phys.Lett.A21:1547-1566,2006 • Neglecting the claim: t(76Ge)> 1.9 x 1025 y • Phys. Rev. D 63 073005 • Possible only if neutrinos are Majorana particlesSchetcher, Valle Phys. Rev. D25 2951 1982 Discrete Conference - Elena Guardincerri

  4. 0 decay and effective mass In the (standard) hypothesis that the process is mediated by the exchange of a light Majorana  the halflife for 0 is: Effective neutrino mass Phase space factor, ~ Q5,can be computed exactly Nuclear matrix element, main source of uncertainty when deriving m from t Discrete Conference - Elena Guardincerri From the measurement of m costraints on the mass hierarchy can be set

  5. 0nbb and mass hierarchy patterns We can express the effective neutrino mass in terms of the measured oscillation parameters and of the unknown lightest neutrino mass: and therefore use the oscillation parameters to constraint mbband mmin degeneracy: m1~ m2~ m3 Next generation 0 experiments Inverted hierarchy: m3<< m2~ m1 mbb in eV best values of oscillation parameters 1s errors for oscillation parameters Normal hierarchy: m1< m2<< m3 Discrete Conference - Elena Guardincerri A. Strumia, F. Vissani, arXiv.org/abs/hep-ph/0606054

  6. The bolometric technique • Principles of operation • DT = E/C • C (heat capacity) low for dielectrics @ low temperatures (T<<1 K) C ~ T3 (Debye) • appreciable DT • CUORE and Cuoricino bolometers are 130TeO2 x-tals • NTD Ge thermistors are used as thermometers • Very good energy resolution • See next slides Heat sink Cuoricino Thermal coupling Energy absorber ( atom x-tal) Thermometer (NTD thermistors in Cuoricino) Discrete Conference - Elena Guardincerri

  7. Bolometric approach: signature Measured: sum of electrons kinetic energy 2nbb with maximum at E ≈ 1/3 Q: uneliminable background • 0nbb peak at Q = Mf – Mi • Mnucl >> me • nuclear recoil can be neglected • peak width due only to detector finite resolution counts(a.u.) A = A/100 sum electron energy / Q A good energy resolution is vital! Discrete Conference - Elena Guardincerri

  8. The candidate isotopes V. Rodin (2009) arXiv:0910.5866 The decay is allowed only for some even-even nuclei for which the “standard” beta decay is forbidden by the law of conservation of energy Nuclei with a high isotopic abundance (or easy enrichment), high M0n, and high Q value (less background from natural radioactivity, phase space factor ~ Q5) are preferable. Discrete Conference - Elena Guardincerri

  9. CUORE and Cuoricino at LNGS 3650 m.w.e. shield against cosmic rays Cuoricino (Hall A) Underground National Laboratory of Gran Sasso L'Aquila – ITALY CUORE Discrete Conference - Elena Guardincerri

  10. The prototype: Cuoricino 5 x 5 x 5 cm3 xtals mixing chamber T ≈ 6 mK • Active mass: • TeO2 40.7 kg • 130Te 11.3 kg Cuoricinotower: 62 TeO2 crystals roman Pb shielding (1 cm lateral) external shields: • 10 cm Pb + 10 cm low act Pb • neutron shield: B-polyethylene • nitrogen flushed anti-radon box • Faraday cage 3 x 3 x 3 cm3 xtals Discrete Conference - Elena Guardincerri

  11. Cuoricino: signal and performances • Shape: • tR ~ 10 ms • tD ~ 500 ms • Amplitude: • 250 mV/MeV • ~ 1 V/MeV after amplification (ms) • 232Th calibration spectrum • ( 3 days of data) • Energy resolution @ 2615 KeV • < DE > = (6.3 ± 2.5) keV • (for the 5 x 5 x 5 cm3 xtals) • Background: (0.153 ± 0.006) c/(keV•kg•y) @ 2527 KeV • (for the 5 x 5 x 5 cm3 xtals) 2615 keV 208Tl single escape double escape Discrete Conference - Elena Guardincerri

  12. Cuoricino performances: the background a region, dominated by a peaks (internalorsurfacecontamination) g region, dominated by g and b events. Highest g line: 2615 keV 208Tl (from 232Th chain) Discrete Conference - Elena Guardincerri

  13. @ 90 % CL Cuoricino results • Rejection of multiple site events • Fit in the 2470-2580 keV region • Flat bkgd + 2505 keV peak + gaussian 0νββ peak Total exposure: 19.75 kg 130Te·y • Efficiency: (82.8 ± 1.1)% large xtals, (79.7 ± 1.4)% small xtals • Various NME calculations: • Simkovic et al. PRC 77 (2008) 045503 • Civitarese et al., JoP: Conference series, 173, 2009, 012012 • Menendez et al., NPA 818 (2009) 139 • Barea and Iachello, PRC 79 (2009) 044301

  14. <mbb>: Cuoricino and CUORE Possible evidence (best value 0.32 eV) Mod.Phys.Lett.A21:1547-1566,2006 Present Cuoricino result degeneracy: m1~ m2~ m3 Inverse hierarchy: m3<< m2~ m1 mbb in eV Normal hierarchy: m1< m2<< m3 In 5 years

  15. Cuore setup and expected performances • 988 TeO2 crystals (750 g each) • 19 towers of 52 crystals each • 741 kg TeO2 • 204 kg 130Te • Expected background: 10-2 c/(keV kg y) • In 5 years of data taking will be sensitive to  ~2.1·1026 y => m~ 35 - 82 meV 80 cm • NMEs from: • Simkovic et al. PRC 77 (2008) 045503 • Civitarese et al., JoP: Conference series, 173, 2009, 012012 • Menendez et al., NPA 818 (2009) 139 • Barea and Iachello, PRC 79 (2009) 044301 19 times Cuoricino

  16. 0nbb: experimental sensitivity • The performances of 0nbb experiments can be compared introducing the detector factor of merit • For a given detector it is the process half-life measurable at 1s C.L. • Emphasized the role of the essential experimental parameters: • M =mass • T = measuring time • h = isotopical abundance • B = background level • e = detection efficiency • DE = resolution x = bb atoms/molecule A = compound molecular mass NA = Avogadro number Discrete Conference - Elena Guardincerri

  17. M = m x 18 T = t x 5 B = b / 20 E = E / 1.5 From Cuoricino to Cuore: the challenges 1 σ CL The non trivial part is achieving 0.01 c/KeV/kg/y. CUORE is 1 ton: this means 10 c/keV/y Discrete Conference - Elena Guardincerri

  18. CUORE vs Cuoricino: improvements Custom refrigerator + 4He-3He dilution unit → No cryogenic liquids for precooling → higher duty cycle Mechanical suspension of the bolometers with complete decoupling from the refrigerator → reduction of vibration induced noise Increased lead thickness around the detector (36 cm min vs 22 cm for Cuoricino) → reduced contribution from environmental radioactivity Strict control of the radioactivity in the experimental setup The 1 Bq 232Th contamination of Cuoricino cryostat resulted in an irreducible background ~ 0.05 c/(keV⋅kg⋅y) Improved cleaning techniques for the construction materials developed in several years of dedicated R&D Discrete Conference - Elena Guardincerri

  19. CUORE: expected background reduction Cuoricino bkgd (0) = 0.15 c/keV/kg/y (main contributions from contaminations in the construction materials) • ϒs from cryostat internal Cu shields (bulk): ~ 30% • TeO2 and Cu surfaces: ~ 70% From tests on crystals and copper surface cleaning: • reduction of crystal surface contamination by a factor ~ 5 • clean or wrap Cu surfaces reduce the continuum background in the 3-4 MeV energy region by a factor ~ 2 • Tests results (above) • Cleaner Cu shields • thicker internal Pb shield • reduced (~ ½) Cu surface area • better detector geometry resulting • in improved coincidence cuts MC simulation 2 - 4 x10-2 c/keV/kg/y • CUORE expected bkgd (0) = 0.01 c/keV/kg/y • Still a factor 2 - 4 to go • Most experimental efforts are now focused on the reduction of surface impurities Discrete Conference - Elena Guardincerri

  20. Cuore status and conclusions • Cuoricino proved the feasibility of CUORE • Multiple tests performed to select the best Cu cleaning method for the frame holding the crystals • Cuore 0 will start next year • 56 new Cuore crystals operated inside Cuoricino cryostat. • CUORE cleaning procedures used • CUORE hut completed • Clean room to be completed in the next month • CUORE cryostat under construction • 500/988 crystals already in Gran Sasso • Data taking is scheduled to start in 2013 Discrete Conference - Elena Guardincerri

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