The European Future of Dark Matter Searches with Cryogenic Detectors H Kraus University of Oxford

1. EURECA The European Future of Dark Matter Searches with Cryogenic Detectors H Kraus University of Oxford

2. European Underground Rare Event Calorimeter Array • Based on CRESST and EDELWEISS expertise, with additional groups joining. • Baseline targets: Ge, CaWO4, etc (A dependence) • Mass: above 100 kg • Timescale: after CRESST-II and EDELWEISS-II • Started 17 March 2005 (meeting in Oxford) • R&D: demonstrate CRESST/EDELWEISS

3. EURECA Collaboration CRESST + EDELWEISS + new forces France CEA/DAPNIA Saclay CEA/DRECAM Saclay CNRS/CRTBT Grenoble CNRS/CSNSM Orsay CNRS/IPNL Lyon CNRS/IAP Paris CERN United Kingdom Oxford (H Kraus, coordinator) Germany MPI für Physik, Munich Technische Universität München Universität Tübingen Universität Karlsruhe Forschungszentrum Karlsruhe

4. Experiments – MSSM Predictions • = 10−6 pb: ~1 event/kg/day ~0.1 now reached • = 10−8 pb: ~1 event/kg/year CDMS-II, CRESST-II and EDELWEISS-II aims • = 10−10 pb: ~1 event/ton/year Next generation requires further x100 improvement!

5. Background Rates Major challenge: typical radioactivity … human body: ~10+6 decays/(kg day) … well above: ~10−4 events/(kg day) Substantial and robust discrimination required: tails of distributions; hard to understand, difficult to simulate with high precision. Entering un-chartered territory: need low event rate in ~keV energy range: atomic x-rays, not MeV as in ν experiments

6. Detection Techniques

7. Cryogenic Techniques Initial recoil energy Displace- ments, Vibrations Athermal phonons Ionization (~10 %) Scintillation (~1 %) Thermal phonons (Heat) Discrimination by combining phonon measurement with measurement of ionization or scintillation Phonon: most precise total energy measurement Ionization / Scintillation: yield depends on recoiling particle Nuclear / electron recoil discrimination.

8. EDELWEISS – Detectors Target: Cyl. Ge crystal, 320 g Ø 70 mm, h = 20 mm Phonon - signal: NTD-Ge (~ 20 mK) Ionisation - signal: Inner disc / outer guard ring

9. Phonon – Ionisation 60Co 252Cf Excellent resolution in both ionisation and phonon signals. Clean γ-calibration data: no event below Q = 0.7.

10. EDELWEISS 1 – Data Data: 22.6 kg.d shown. Probable surface event contamination at Q<0.7 Challenge: less than perfect charge collection for surface events

11. Identification of Backgrounds Germanium Surface Events Example of 3rd population, affecting rejection efficiency. Quality of Rejection Importance of selection variable having good separation and resolution. More Rejection Signatures Recoil spectrum, coincidence, charge, scintillation, type of recoiling nucleus, etc.

12. EDELWEISS 21×320g Ge with NTD 7×400g Ge with NiSb LSM (4800 m.w.e)

13. EDELWEISS – New Cryostat Up to 120 Detectors

14. EDELWEISS – Shielding • 20 cm lead • 50 cm PE • Muon veto

15. March 2005 March 2005 May 2005 Edelweiss II installation at LSM May 2005: lead, upper and lower PE shields completed. Start μ-VETO installation. Summer: installation of cryostat. Autumn: first pulses.

16. CRESST – Detectors normal-conducting Resistance [m] R T super-conducting heat bath thermal link thermometer(W-film) absorbercrystal Width of transition: ~1mKSignals: few  KStablity: ~  K • Particle interaction in absorber creates a temperature rise in thermometer which is proportional to energy deposit in absorber Temperature pulse (~6keV)

17. Phonon – Scintillation separate calorimeter as light detector W-SPT 300 g CaWO4 W-SPT light reflector Discrimination of nuclear recoils from radioactive backgrounds (electron recoils) by simultaneous measurement of phonons and scintillation light proof of principle high rejection:99.7% > 15 keV99.9% > 20 keV Energy in light channel keVee] Energy in phonon channel [keV]

18. 300g Detector Prototype CRESST II: 33 modules; 66 readout channels

19. Run 28: Low Energy Distribution No Neutron Shield 10.72 kg days 90% of oxygen recoils below this line.Rate=0.870.22 /kg/day compatible with expec-ted neutron background (MC). 90% of tungsten recoils Q = 40 below this line. No events

20. Upper Limits on Scalar WIMP-Nucleon Cross Section Cryogenic Detectors only

21. Expected Recoil Spectrum in GS MC simulation of dry concrete (Wulandari et al) Contribution of W recoils negligible for E > 12 keV σ A2 for WIMPs with spin-independent interaction WIMPs dominantly scatter on W (A=184) nuclei Neutrons mainly on oxygen

22. Quenching Factor Measurement Ion source PTFE reflector Deflection plate for ion type selection W, O, Ca ions PMT UV Laser collimator CaWO4 crystal target • UV Laser desorbs singly or doubly charged ions from almost any material. Acceleration to 18 keV (or 36 keV for doubly charged). • Mount CaWO4 crystal on PMT at end of flight tube and record single photon counts with fast digitizer.

23. Quenching Factors for CaWO4

24. Upgrade • Read out electronics:66 SQUIDs for 33 detector modules and DAQ ready • Neutron shield: 50 cm polyethylen (installation complete) • Muon veto: 20 plastic scintillator pannels outside Cu/Pb shield and radon box. Analog fibre transmission through Faraday cage (ready) • Detector integration in cold box and wiring (entering fabrication stage)

25. High Energy Performance • Excellent linearity and energy resolution at high energies • Perfect discrimination of , from s • Identification of alpha emitters (internal, external)

26. Decay of “stable” Tungsten-180 144Nd 180W 152Gd 147Sm

27. Decay of “stable” Tungsten-180 180W

28. Evolution of Sensitivity

29. Signatures • Recoil energy spectrum Energy resolution • Nuclear (not electron) recoils Discrimination • Coherence: μ2A2 dependence Multi-target • Absence of multiple interactions Array • Uniform rate throughout volume Large Array • Annual modulation (requires many events)

30. EURECA Tasks Detector Development: improving rejection, optimizing size, mass production issues. Readout and Electronics: scalability. Cryogenic Environment: size, radiopurity, uptime. Neutron and Muon Backgrounds: measurement, simulation and shielding design. Extreme Low-background Materials: selection of materials, processing, handling, etc.

31. EURECA Strategy Some aspects covered by ILIAS working groups. Main thrust: demonstrate current experiments. EDELWEISS: new (larger) cryostat; improved shielding; 8kg Ge by end of 2005; reduction of surface events expected from improved radiopurity and/or use of NbSi sensors. Up to ~30kg target possible in cryostat. CRESST: new (66 channel) SQUID readout system; improved shielding; few kg CaWO4 by end of 2005; continuous improvement scintillation signals. Up to ~10kg target possible in present cryostat.

32. Summary CRESST and EDELWEISS are on track to reaching LHC-relevant sensitivity; but major improvements w.r.t. present achievements will have to be shown. Cryogenic Detector Technology with nuclear recoil identification has the necessary potential for these improvements. The EURECA collaboration builds on CRESST and EDELWEISS experience aiming towards a European multi-target array for direct dark matter searches.