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CRESST Cryogenic Rare Event Search with Superconducting Thermometers

CRESST project at LNGS focuses on Cryogenic Dark Matter search using CaWO4 crystals as targets in a shielded setup for low-mass WIMP detections. The detectors with excellent energy resolution enable discrimination of nuclear recoils from background, allowing for precise energy calibrations. Ongoing since 2009, the project utilizes up to 33 detector modules for nuclear recoil discrimination and background estimation.

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CRESST Cryogenic Rare Event Search with Superconducting Thermometers

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  1. CRESST Cryogenic Rare Event Search with Superconducting Thermometers CRESST Max-Planck-Institut für Physik University of Oxford Technische Universität München Laboratori Nazionali del Gran Sasso Universität Tübingen Cryogenic Dark Matter search Located in Hall A of LNGS Scintillating CaWO4 target crystals Up to 33 crystals in modular structure (10 kg target mass) Different materials possible

  2. Which recoil nucleus is seen in CaWO4 assuming σ ∝ A2 Threshold 10keV • For small WIMP masses <10 GeV only O recoils above threshold • Ca is important around 10 GeV • For large masses W dominates due to σ∝A2

  3. CRESST set-up at LNGS shielding: • underground laboratory • 45 cm PE (12 tons) • muon-veto • radon box • 20 cm lead (24 tons) • 14 cm copper (10 tons) • use only radio-pure materials

  4. Half Cu/Pb schield closed Coldbox closed

  5. normal-conducting Resistance [m] R T super-conducting CRESST type cryogenic Detectors heat bath thermal link thermometer(W-film)‏ absorbercrystal SQUID based read out circuitWidth of transition: ~1mK, keV signals: few μK Longterm stablity: ~ μK Temperature pulse Advantages of technique: - measures deposited energy independent of interaction type- Very low energy threshold and excellent energy resolution-Many different target materials E

  6. β+γ 300g scintillaingCaWO4 crystal α O W CRESST-II Detectors Discrimination of nuclear recoils from radioactive backgrounds by simultaneous measurement of phonons and scintillation light W sensor Light detector Light reflector (scintillating) W sensor Identification of recoiling nucleus possible

  7. 300 g CRESST-II Detector Module The phonon detector: 300 g cylindrical CaWO4 crystal. Evaporated tungsten thermometer with attached heater. Clamps not scintillating The light detector:Ø=40 mm silicon on sapphire wafer.Tungsten thermometer with attached aluminum phonon collectors and thermal link. Part of thermal link used as heater CRESST-II: up to 33 detector modules

  8. Spectral features at low Energies Cu Kα 8.1 keV found @8.2 keV 41Ca 3.61 keV found @3.6 keV 210Pb 46.5 keV @ 46.5 keV • Very precise energy calibration • Lines down to 3.6 keV identified with excellent energy resolution of 300 eV.

  9. Present run All results preliminary • running since summer 2009 • 10 detectors running (1 ZnWO4) • Clamps not covered with scintillator • data analysis is still in progress • Data discussed are from 9 CaWO4 detectors ( about 400 kgd)

  10. Stability

  11. neutron calibration γ + β band O-recoil-band Lines calculated from e-resolution of light detector and known quenching factor Excellent nuclear recoil discrimination

  12. Data γ + β band α band O-recoil-band W-recoil-band Pb α clamp not scint. 210Po 206Pb (104 keV) + α (5.4 Me)‏ Pb α crystal

  13. What are these events in O-band ? Try to estimate background Check for coincidences Neutrons ? α leakage ? Low mass WIMPs ?

  14. Neutrons ? No double coincidences 2 tripple coincidences : Orecoil + Orecoil + 1.8MeV gamma Orecoil + 30 KeV gamma + 1.18MeV gamma • O+O coincidences can only come from neutrons • When there are coincidences there are also single O recoils • Goal: Estimate single oxygen recoils from observed coincidences. Ratio of coincidence/single depends on the source of neutrons. Neutrons from sf or alpha-n mostly n-n, few n-gamma very few triple coincidences and none with MeV gammas unlike observed 2 events Neutrons induced by muons in Pb/Cu shield shower like events with high multiplicity and high energy gammas, similar to observed 2 events.

  15. Multiplicity of oxygen recoils in signal region Multiplicity with neutron source Multiplicity from muon induced neutrons Only very view oxygen recoils seen in coincidence in 3 detectors, no high energy gammas involved Oxygen recoils seen in coincidence in a large number of detector modules together with high energy gammas in others. Similar to the observed events Coincident/single = 17/8 = 2.1 ±0.9 Single neutron background of 0.94±0.83 events in O band

  16. Degraded alphas from external contamination in clamps • Discrete alpha lines from contamination in crystal are no problem • Degraded alphas with continuous energy distribution down to lowest energies from external contamination in clamps

  17. Estimation of α background in oxygen band • Detector with highest external alpha background • Also highest 206Pb recoil background, with long tail extending from 100 keV down to 40 keV Alpha band Oxygen band Signal region 206Pb recoils • Background of degraded external α's from contaminated clamps. • Oxygen and α band partially overlap and some α's may leak into signal band. • Estimate dN/dE in overlap free region of alpha band and then compute expectation in oxygen band assuming constant dN/dE.

  18. Estimation of Background of degraded alphas Nfound: Number of alpha events found in reference region of alpha band Nproj: The projected number of alpha events in signal region of the oxygen recoil band Estimate of α background in signal energy range of oxygen band: 6.93 counts

  19. Summary of Background Estimates Neutron background 0.9 Alpha background: 6.9 Leakage from gamma band: 0.9 Sum of background estimates: 8.7 Observed oxygen recoil signals : 32 background estimate of 8.7 ± 1.4 not enough to explain observed 32 signals, leaving space for a light WIMP m about 15 GeV or less σ about a few times 10-5pb

  20. Inelastic Dark Matter preliminary preliminary preliminary preliminary

  21. Conclusions • CRESST detectors are very powerful and able to perform precision measurements • Inelastic Dark Matter scenario becomes very unlikely to explain the DAMA result • Neutron background is negligible and can not explain our signals in oxygen band • Background from degraded alphas is less then observed signals in oxygen band, a precise estimate is difficult. • If the alpha estimate is roughly O.K, there is space for a light WIMP • A new run with strongly reduced alpha background is the next step. It should help to pin down the nature of the observed signals with high confidence.

  22. Thank You

  23. Detector Performance in wide energy range • Enormous dynamic range • Excellent linearity and energy resolution in whole energy range • Perfect discrimination of β and γ from α’s • Identification of alpha peaks from emitters in crystal β+γ α peaks Nuclear recoils from neutrons or WIMPs

  24. Identification of a WIMP-signal The trouble starts if you see a signal • Annual modulation • you have to prove that your background is constant in time ! • and your detector runs stable • Use different target nuclei in same detector • cross section for background depends differently on target nucleus (mass) than WIMP-scattering • unique feature of CRESST Detectors

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