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EDELWEISS-II : Status and future

EDELWEISS-II : Status and future. Véronique SANGLARD CNRS/IN2P3/IPNLyon sanglard@ipnl.in2p3.fr http://edelweiss.in2p3.fr. Outline. Context EDELWEISS-I Run with heat trigger Final results Background studies EDELWEISS-II Improvements Present and future runs Conclusion.

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EDELWEISS-II : Status and future

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  1. EDELWEISS-II : Status and future Véronique SANGLARD CNRS/IN2P3/IPNLyon sanglard@ipnl.in2p3.fr http://edelweiss.in2p3.fr

  2. Outline • Context • EDELWEISS-I • Run with heat trigger • Final results • Background studies • EDELWEISS-II • Improvements • Present and future runs • Conclusion Dark Matter 2006 – Marina del Rey, California « EDELWEISS-II : Status and future » Véronique SANGLARD

  3. The EDELWEISS Collaboration *Expérience pour DEtecter Les WIMPs En SIte Souterrain (Underground experiment to detect WIMP) • CEA-Saclay DAPNIA/DRECAM • CNRS/CRTBT Grenoble • CNRS/IN2P3/CSNSM Orsay • DUBNA (Russia) • FZK/Univ. Karlsruhe (Deutschland) • CNRS/INSU/IAP Paris • CNRS/IN2P3/IPN Lyon • CNRS – CEA/Laboratoire Souterrain de Modane • 1700 m depth under the Fréjus tunnel (4800 we) • 4 µ/m²/d (106 less than at the surface) Dark Matter 2006 – Marina del Rey, California « EDELWEISS-II : Status and future » Véronique SANGLARD

  4. Heat and Ionization Ge detectors Center electrode Guard ring 7 cm m=320g Ionization threshold Neutrons 73Ge(n,n',γ) Gammas Heat Reference electrode • Simultaneous measurement of • Heat @ 17 mK with Ge/NTD sensor • Ionization @ few V/cm with Al electrodes • Different charge/heat ratio for nuclear and electron recoils (WIMP and neutron have lower light/charge than γs, βs ) • Discrimination event-by-event of electron recoils (main background) • EI/ER = 0.3 for nuclear recoils • EI/ER = 1 for electronic recoils Thermometer (Ge NTD) Fiducial volume(≈ 55%) Ge crystal Guard Electrodes Center electrode Ionization guard Ionization center Dark Matter 2006 – Marina del Rey, California « EDELWEISS-II : Status and future » Véronique SANGLARD

  5. 2003 data taking with heat trigger • New data taking with trigger on heat signal • Improved efficiency at low energy (50 % at 11 keV) • Fiducial exposure: 22 kg.d • Stable behavior over 4 months • 18 nuclear recoil candidates > 15 keV • Possible backgrounds • Residual neutron flux • 1 n-n coincidence observed • 2 single expected by MC • Surface electron recoils • Miscollected charge events at low energy • Not visible in coincidence events • Further background studies Dark Matter 2006 – Marina del Rey, California « EDELWEISS-II : Status and future » Véronique SANGLARD

  6. Final results of EDELWEISS-I (1) • Final results: 62 kg.d (fid. exp.) • 50% trigger efficiency at 15 keV • 40 nuclear recoil candidates > 15 keV (only 3 with 30 <ER< 100 keV) • Unknown background • Used method developed by S. Yellin to derive exclusion limits (as CDMS) *(PRD 66,032005 (2002)) • No background subtraction • V.Sanglard et al., PRD 71,122002 (2005) • Experiment stopped in March 2004 Dark Matter 2006 – Marina del Rey, California « EDELWEISS-II : Status and future » Véronique SANGLARD

  7. Exclusion limit – Spin Independent • EDELWEISS starts to explore some optimistic SUSY models • Best sensitivity : 1.5x10-6 pb @ 80 GeV/c² • Need a gain in sensitivity of a factor 100 – 10000 (EDW-II, EURECA) • This gain depends on improvements • On background discrimination • On mass Dark Matter 2006 – Marina del Rey, California « EDELWEISS-II : Status and future » Véronique SANGLARD

  8. Exclusion limit – Spin Dependent • 7.8 % of 73Ge (J=9/2) in natural Ge • Spin on neutron or proton • results present in (ap, an) diagram • ap, an = effective coupling between WIMP and proton, neutron • For each WIMP mass, the constraint is Optimisitic SUSY models are a factor 100 below Direct detection experiments not yet competitive with indirect detection (SK) Dark Matter 2006 – Marina del Rey, California « EDELWEISS-II : Status and future » Véronique SANGLARD

  9. g-ray background in EDELWEISS-I • Before nuclear recoil selection, rate in detectors is ~1.5 event/kg/day/keV at low energy • At high energy, spectrum shape and rate consistent with simulations of the measured U/Th contamination in the bulk of the Cu shielding • Room for improvement in EDELWEISS-II, where this Cu is not used Dark Matter 2006 – Marina del Rey, California « EDELWEISS-II : Status and future » Véronique SANGLARD

  10. Neutron background in EDELWEISS-I • Measurement of the neutron flux with E >1 MeV : 1.1  0.1(stat) 10-6 n/cm²/s • Good understanding of neutron propagation in the setup (agreement between simulated and experimental spectrum) • Determination of single rate : ~2 nuclear recoil expected in 62 kg.d (ambient radioactivity + U contamination of copper and lead) • 1 n-n coincidence observed in 62 kg.d • Not a strong constraint on the single neutron rate in the data • Expected ratio double/single ~1/10 • 1 n-n  1 – 40 single with ER>15 keV @ 90% C.L. • Indistinguishable from the miscollected events in the nuclear recoil band Dark Matter 2006 – Marina del Rey, California « EDELWEISS-II : Status and future » Véronique SANGLARD

  11. Surface backgrounds in EDELWEISS-I • Peak at E=5.3 MeV • a’s from 210Po? • Q=0.3  a decays near surfaces • Rate ~ 400 /m²/j • As expected, non-fiducial part more exposed to a flux • Very likely due to 210Pb on Cu or Ge surfaces • No 206Pb recoil peak at 100 keV observed as heat-only events : 210Pb implanted in Cu, not Ge. • Rate of 0.3<Q<1.0 events at low energy consistent with surface b’s expected in 210Pb hypothesis • (but does not exclude possiblecontribution from 14C) • By removing Cu covers between detectors • Possible gain • Better identification by coincidences E = 5.3 MeV, Q = 0.3 1 Dark Matter 2006 – Marina del Rey, California « EDELWEISS-II : Status and future » Véronique SANGLARD

  12. EDELWEISS-II in few words • Installation in the LSM started summer 2004 • 1rst funded stage : 28 detectors • 21*320g optimized Ge/NTD detectors and holders • 7*400g Ge/NbSi detectors with active surface events rejections • First cryogenic test with bolometers jan 2006 • Commissioning run with 8 bolometers • New electronic and acquisition systems: square modulation, continuous digitization close to the readout, optical fibers and numerical trigger • Goal *100 in sensitivity : • w-n10-8 pb • 0.002 evt/kg/day (ER>10keV) Dark Matter 2006 – Marina del Rey, California « EDELWEISS-II : Status and future » Véronique SANGLARD

  13. EDELWEISS-II improvements – Cryostat • Reversed geometry • Nitrogen free : 3 Pulse tube (50K and 80K screens) and 1 He cold vapor reliquefier (consumption  0) • Large volume 50l  Self shielding • Up to  120 detectors More statistics • Compact and hexagonal arrangement  More coincidence (n bkg) Dark Matter 2006 – Marina del Rey, California « EDELWEISS-II : Status and future » Véronique SANGLARD

  14. EDELWEISS-II improvements – Backgrounds • Radiopurity • Dedicated HPGe detectors for Systematic checks off all materials • Clean Room (class 100 around the cryostat, class 10000 for the full shielding • Deradonized air (from NEMO3) (0.1 Bq/kg) • 20 cm Pb shielding • Neutron Shielding • EDW-I : 30cm paraffin • EDW-II : 50 cm PE and better coverage • µ veto (99% coverage) Dark Matter 2006 – Marina del Rey, California « EDELWEISS-II : Status and future » Véronique SANGLARD

  15. EDELWEISS-II improvements – Ge/NTD detectors • Developed by CEA Saclay and Camberra-Eurisys • Amorphous Ge and Si sublayer (better charge collection for surface events) • Optimized NTD size and homogeneous working T (16-18 mK) : sub keV resolution • New holder and connectors (Teflon and copper only) Dark Matter 2006 – Marina del Rey, California « EDELWEISS-II : Status and future » Véronique SANGLARD

  16. EDELWEISS-II improvements – surface events • Important limitation to the sensitivity : due to trapping and recombination, surface events are miscollected and can mimic nuclear recoils • 2 approaches : • Passive rejection : improve the charge collection for surface event • Physics of the charge collection, trapping, surface charge, regeneration • Physics of the Ge and Si amorphous sublayer • Detectors with thick electrodes • Active rejection : identification of the surface events • Pulse shape analysis of the charge signals (but high bandwidth  low noise)  localization of the event • Interdigitzed electrodes • Detectors sensitive to athermal phonons  Ge/NbSi detectors Dark Matter 2006 – Marina del Rey, California « EDELWEISS-II : Status and future » Véronique SANGLARD

  17. Identification of surface events with Ge/NbSi detector Identification of near surface events using athermal phonon measurement with NbSi thin film thermometers • Heat and ionization Ge detectors • Enhancement of athermal signals for near surface events • Thermal signals proportionnal to the deposited energy Improvement of a factor 20 Fiducial volume reduction of 10 % Before rejection After rejection (1mm cut) Dark Matter 2006 – Marina del Rey, California « EDELWEISS-II : Status and future » Véronique SANGLARD

  18. EDELWEISS-II present status • 8 bolometers : • 2*320g Ge/NTD with EDW-I holder • 2*320g Ge/NTD with EDW-II holder and teflon clamp • 2*320g Ge/NTD with EDW-II holder and Cu springs • 1*IAS 50g heat and light detector (Al2O3) • 1*200g Ge/NbSi • Goals : Validation of the microphonics (pulse tube decoupling system), new holders and new comb connectors for Ge/ntd, new electronics scheme, new acquisition system… Dark Matter 2006 – Marina del Rey, California « EDELWEISS-II : Status and future » Véronique SANGLARD

  19. EDELWEISS-II future • Feb-march 2006 : present run =commissioning • April 2006 : EDELWEISS-II 28 detectors (21 Ge/NTD and 7 Ge/NbSi) + IAS detector (heat and light) • 2006 : Approval of the 2nd stage : 120 detectors • In term of sensibility : • EDELWEISS-II : 1 evt/kg/year • EURECA : 10 evt/kg/an Dark Matter 2006 – Marina del Rey, California « EDELWEISS-II : Status and future » Véronique SANGLARD

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