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

Based on CRESST and EDELWEISS expertise, with additional groups joining. Baseline targets: Ge, CaWO 4, 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 European Underground Rare Event Calorimeter Array

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

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!

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

Detection Techniques

Cryogenic Techniques Initial recoil energy Displace- ments, Vibrations Athermal phonons Ionization (~10 %) Thermal phonons (Heat) Scintillation (~1 %) 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.

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

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

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

Identification of Backgrounds Germanium Surface Events Example of 3 rd 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.

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

EDELWEISS – New Cryostat Up to 120 Detectors

EDELWEISS – Shielding 20 cm lead 50 cm PE Muon veto

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.

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

Phonon – Scintillation Discrimination of nuclear recoils from radioactive backgrounds (electron recoils) by simultaneous measurement of phonons and scintillation light separate calorimeter as light detector light reflector W-SPT 300 g CaWO 4 proof of principle Energy in light channel keV ee ] Energy in phonon channel [keV] high rejection: 99.7% > 15 keV 99.9% > 20 keV

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

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

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

Expected Recoil Spectrum in GS Contribution of W recoils negligible for E > 12 keV σ  A 2 for WIMPs with spin-independent interaction WIMPs dominantly scatter on W (A=184) nuclei Neutrons mainly on oxygen MC simulation of dry concrete (Wulandari et al)

Quenching Factor Measurement PMT UV Laser W, O, Ca ions CaWO 4 crystal PTFE reflector collimator Ion source UV Laser desorbs singly or doubly charged ions from almost any material. Acceleration to 18 keV (or 36 keV for doubly charged). Mount CaWO 4 crystal on PMT at end of flight tube and record single photon counts with fast digitizer. Deflection plate for ion type selection target

Quenching Factors for CaWO 4

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)

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

Decay of “stable” Tungsten Sm 152 Gd 144 Nd 180 W

Results from four runs (28.62 kg days) Half life T 1/2 = (1.8±0.2) × years Energy Q = (2516.4±1.1 (stat.)±1.2(sys.)) keV Decay of “stable” Tungsten-180

Evolution of Sensitivity

Signatures 1.Recoil energy spectrumEnergy resolution 2.Nuclear (not electron) recoilsDiscrimination 3.Coherence: μ 2 A 2 dependenceMulti-target 4.Absence of multiple interactionsArray 5.Uniform rate throughout volumeLarge Array 6.Annual modulation (requires many events)

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.

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 CaWO 4 by end of 2005; continuous improvement scintillation signals. Up to ~10kg target possible in present cryostat.

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.