Esperimenti bolometrici al Gran Sasso: CUORE e CRESST

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1 Esperimenti bolometrici al Gran Sasso: CUORE e CRESST Marco Vignati 24 Ottobre 2011

2 0νDBD in Theory Nuclear process: (A,Z) (A,Z+2) + 2 e - Can only happen if lepton number is not conserved. The decay probability depends on the effective Majorana mass m of the neutrino between the two electron vertexes. The measurable quantity is the half-life ( 0 1/2 ) of the decay: Phase space factor: ~ Q 5 Nuclear Matrix element Effective neutrino mass 1 ( 1/2 0 ) = G(Q, Z) M nucl 2 m 2 m = F 2 N m 2 e 2

3 0νDBD in Experiments Experiments measure the sum of the kinetic energies of the two emitted electrons. Signature: monochromatic line at the Q-value of the decay. Sensitivity ( S 0 ): lifetime corresponding to the minimum number of detectable events above background at a given C.L.: Isotopic abundance S 0 = ln 2N A a A Detector mass (kg) Mt B E Measurement time (y) 1/2 Efficiency Atomic mass Background (counts/kev/kg/y) 3 Energy Resolution (kev)

4 Bolometric technique Particle energy converted into phonons temperature variation. TeO2 crystals (dielectric, diamagnetic) detector = source Low crystal heat capacitance and low base temperature to see small temperature variations ΔT ~ E/C Heat bath ~ 10 mk Weak thermal coupling Energy release Thermometer: NTD Ge thermistor R ~ 100 MΩ Absorber Crystal TeO2 C ~ J/K Detector response in this configuration: ~ 0.1 mk / MeV (2527 kev) ~ 5 kev FWHM 4

5 The demonstrator: Cuoricino Active mass: TeO2: 40.7 Kg 130 Te: 11.3 Kg 128 Te: 10.5 Kg 11 modules: 4 detectors of 5x5x5 cm 3 = 790 g each 2 modules: 9 detectors of 3x3x6 cm 3 = 330 g each 2 enriched in 128 Te (82%) 2 enriched in 130 Te (75%) Data taking Installed in a dilution refrigerator: Inner shield: - 1cm Roman Pb A ( 210 Pb) < 4 mbq/kg External Shield: - 20 cm Pb - 10 cm Borated polyethylene Nitrogen flushing to avoid Rn contamination. 5

6 Cuoricino limit on 0νDBD half-life Analyzed statistics: 0νDBD Half-life limit (90% C.L): Effective neutrino mass limit: Energy resolution: Rate [counts/ (1 kev)] Astropart. Phys. 34 (2011) Co γ + γ 0νDBD Best Fit 68% C.L. 90% C.L Energy [kev] 6 NME bibliography: 1 Šimkovic et al., PRC 77 (2008) Civitarese et al., JoP:Conference series 173 (2009) Menéndez et al., NPA 818 (2009) Barea and Iachello, PRC 79 (2009)

7 988 TeO2 crystals Ton scale bolometric experiment for the search of neutrinoless double beta decay of 130 Te 19 towers of 52 crystals each 750 g per crystal 741 kg TeO2 592 kg Te (206 kg 130 Te) 149 kg O Start data taking in 2014 CUORE Laboratori Nazionali del Gran Sasso, Italy, a natural shield of 1400 m of rock (equivalent vertical depth: 3100 m.w.e.) 7

8 Dark matter particles To fulfill cosmological observations dark matter particles must have these properties: Stable or long-lived Neutral Reproduce ΩDMh 2 ~ 0.1 Energy density ρ ~ 0.3 GeV/cm 3 Maxwellian velocity distribution with v0 ~ 220 km/s WIMPs (Weakly interactive massive particles) of mass ~ 100 GeV, are the golden candidates. the supersymmetric particle χ, the neutralino, would have such properties. 8

9 Detection principle Elastic scattering off nuclei, measure nuclear recoil energy Enr: χ χ (A,Z) E=0 (A,Z) Enr For mχ = 100 GeV and A = 100: σ = cm 2 R = events / (kg day) Enr = 0-25 kev Nuclear recoil detection via: ionization scintillation heat 9

10 The WIMP signal / background Exponential like shape, increasing at low E (similar to many bkgs...) Total Rate [evt/kg/day] Si (A = 28) Ar (A = 40) Ge (A = 73) Xe (A = 131) 10 5 σ χn = cm E th - Nuclear Recoil Energy [kev] Demands O(keV) thresholds and backgrounds close to zero. All experiments operated in low radioactivity environments and deep underground. 10

11 Annual modulation Earth velocity combines to solar system velocity Dark matter wind velocity is modulated: 60º 232 km/s Affecting the detection rate consistently: Sun Earth 30 km/s Distinctive modulation signal features: T = 1 year t0 = 2 nd June 11

12 CaWO4 crystals (300g) operated as bolometers: counts per kg d pb in [12,40] kev detect also scintillation light to discriminate nuclear recoils Multi-target: sensitive to different WIMP masses: 5 10 W O Ca phonon detector light detector reflecting scintillating housing W thermometer Light absorber CaWO 4 target W thermometer WIMP mass [GeV] 3 12

13 CRESST - setup 10 mk operating temperature 18 modules: 8 active 330g CaWO4 crystals 1 CaWO4 bad resolution 1 ZnWO4 not well working 10 CaWO4 with light detector not working. Other 12 modules used to reject multi-hit events γ-calibrations with 57 Co and 232 Th 13

14 CRESST - discrimination Light Yield (LY) = L / E characteristic of the event type: LY(e-recoils) :1 (by def.) LY(α) ~ 0.22 LY(O) ~ 0.10 LY(Ca) ~ 0.06 LY(W) ~ 0.04 Data from one module e - /γ band 80% bands accepted events Acceptance region: 10 ~<Energy<40 low bound depends on the module (defined as: 1 γ event leakage) 67 accepted events in the 8 modules (730 kg days) 14

15 CRESST - Global fit Global likelihood fit including the pdfs of all contributions. Two minima found at mwimp=25 GeV and mwimp=12 GeV. accepted events / kev total WIMP signal γ bck Pb recoil bck α bck neutron bck M1 M2 e / events events neutron events Pb recoils signal events m WN GeV pb Energy [kev] 4.7σ for mwimp=25 GeV and 4.2σ for mwimp=12 GeV 15

16 CRESST - result WIMP-nucleon cross section [pb] M2 M1 CRESST 1σ CRESST 2σ CRESST 2009 EDELWEISS-II CDMS-II XENON100 DAMA chan. DAMA CoGeNT WIMP mass [GeV] 16