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EDELWEISS-II : Status and first results A new generation of background-free bolometers for WIMP search. Outline. Reminder on dark matter direct search From EDELWEISS-I to EDELWEISS-II New InterDigit detectors Some current results Outlook. Scatt. WIMP. WIMP. Xe. Ge. Recoil nucleus.

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Outline

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  1. EDELWEISS-II : Status and first resultsA new generation of background-free bolometers for WIMP search

  2. Outline • Reminder on dark matter direct search • From EDELWEISS-I to EDELWEISS-II • New InterDigit detectors • Some current results • Outlook TAUP09 - Roma - 07/03/2009 - SANGLARD Véronique

  3. Scatt. WIMP WIMP Xe Ge Recoil nucleus WIMPs direct search • WIMPs forming our Galactic halo • v ~ 200 km/s • 10 - 1000 GeV/c² mass • exponential spectrum • SUSY neutralino prediction < 1 coll/kg/month • Experimental challenges • Low energy threshold ~ few keV’s • Large mass ~ 1 ton • Low background < 102 /kg/day •  radioactivity (electron recoils) •  radioactivity (surface electron recoils) • Neutron scatterings (nuclear recoils) • Principle • Detection of the energy deposition in a target • Nuclear recoil : 10-50 keV TAUP09 - Roma - 07/03/2009 - SANGLARD Véronique

  4. Ionization threshold Neutrons 73Ge(n,n',γ) Gammas EDELWEISS-I : Heat-Ionization NTD sensors • Ionization measurement @ few V/cm • Heat measurement (NTD sensor) @ 20 mK • Discriminating variable between electronic and nuclear recoils : « Q » ~ ionization/heat • « Edelweiss-I » detectors  « NTD » type detectors : plain Al electrodes Center electrode Guard ring m=320g TAUP09 - Roma - 07/03/2009 - SANGLARD Véronique

  5. 210Pb 210Po β a 206Pb EDELWEISS-I results (2005) • Charge collection deficit of electron like events : « surface » events • 210Pb surface contamination can explain observed alphas at high energy and betas at low energy PRD 71, 122002 (2005) Astro. Part. 28, 143 (2007)  Bad charge collection TAUP09 - Roma - 07/03/2009 - SANGLARD Véronique

  6. The EDELWEISS-II collaboration • CEA Saclay (IRFU and IRAMIS) Detectors, electronics, acquisition, background, analysis E. Armengaud, O. Besida, G. Gerbier, M. Gros, X-F. Navick, B. Paul, S. Hervé, M. Karolak, (M. Chapellier), P.Pari Post-doc: M Hannewald Thesis : A. Chantelauze (cotutelle FZK), J Domange BDI • CNRS, CSNSM Orsay Detectors, cabling, cryogenics, analysis L. Berge, A. Broniatowski, D. Carré, S. Collin, L. Dumoulin,, F. Lalu, S. Marnieros Post-doc : E. Olivieri Thesis : O. Crauste, J Domange BDI • CNRSIPN Lyon Electronics, background, analysis, detectors C Augier, F. Charlieux, M. De Jésus, P. Di Stefano, J. Gascon, A. Juillard, V. Sanglard, L. Vagneron Thesis : M-A. Verdier, S. Scorza • CNRSI Néel Grenoble Cryogenics, electronics A. Benoit • FZ/ Universität Karlsruhe Muon Veto and neutron counter, background J. Blümer, K. Eitel Post doc : V Koslov Thesis : H. Kluck, A. Chantelauze (cotutelle FZK) • JINR Dubna Monitoring of neutron and radon backgrounds E. Yakushev, S. Semikh Thesis : A. Lubashevski, L. Perevozchikov • Oxford University High impedance sensors, wiring H Kraus, S Henry Thesis : S Kierkby, TAUP09 - Roma - 07/03/2009 - SANGLARD Véronique

  7. Polyethelene shield Pb shield cryostat Muon Veto Neutron counter The EDELWEISS-II experiment • Operated at the Underground Laboratory of Modane (4μ/d/m2) • Shieldings : • Clean room + Radon-free air • Active muon veto (> 98% coverage) • PE shield 50cm • Lead shield 20cm •  background reduced by ~2 wrt EDW1 • Cryogenic installation (~ 20 mK) : • Reversed geometry cryostat • Use of pulse tubes • Can House up to 40kg of detectors • Facilities : • Remotely controlled sources for calibrations + regenerations • Detector storage & repair within the clean room • 12 cool-downs already operated TAUP09 - Roma - 07/03/2009 - SANGLARD Véronique

  8. EDELWEISS-II 93.5 kgd The issue of surface interactions • GGA1 « NTD type » detector equiped with 210Pb nano source : • Incomplete charge collection • « Leakage » down to the nuclear recoil region Preliminary • EDW-II 2008 data : • 11 detectors with < 30 keV thresholds • 93.5 kg.d in 4 months • Threshold chosen a priori • 3 events in nuclear recoil band • Beta bkg reduced wrt EDW-I but still insufficient to reach 10-8 pb TAUP09 - Roma - 07/03/2009 - SANGLARD Véronique

  9. Rejecting surface events with interleaved electrodes The “ID” (InterDigit) detector • Keep the EDW-I phonon detector • Modify the E field near the surfaces with interleaved electrodes • Use ‘a’ and ‘c’ signals as vetos against surface event ‘b’ electrodes (+2V) collecting ‘a’ electrodes (-1V) field shapping « VETO » Surface Guard Volume ‘d’ (-2V) ‘c’ (+1V) • First detector built 2007 • 1x200g + 3x400g tested in 2008 • 10x400g running since spring 2009 TAUP09 - Roma - 07/03/2009 - SANGLARD Véronique

  10. An outstanding surface event discrimination « single-side » surface events E1=0 Beta calibration (210Pb) 200g detector « 3-electrodes » surface events 46 keV gamma-ray line fiducial volume events E1=E2 Preliminary • A combination of cuts is made • on the signals of electrodes • Example : cut on the difference of signals between 2 collecting electrodes • Surface and volume events are completely separated ! • From β calibration : overall rejection ~ 1/105 ½( ) ½ ( ) TAUP09 - Roma - 07/03/2009 - SANGLARD Véronique

  11. ID fiducial volume • Approx. ~ 20 kg.d of background data with two 400g detectors (2008 data) • Cosmogenic lines :68Ge and 65Zn lines at ~ 10 keV • Background electron recoils are distributed homogeneously in the volume of the cristal • Fiducial volume = 160g, primarily limited by the guard regions • Result confirmed with neutron calibration measurements 400g detector background runs Preliminary σion = 300 eV 10.37 keV 8.98 keV Preliminary ½( ) Fiducial volume cut TAUP09 - Roma - 07/03/2009 - SANGLARD Véronique

  12. IDs : overall background rejection performances arXiv:0905.0753 -equivalent to ~103 kgd -equivalent to 3x104 kgd 0 events TAUP09 - Roma - 07/03/2009 - SANGLARD Véronique

  13. WIMP-nucleon cross-section (pb) WIMP mass (GeV/c²) Physics runs with IDs • 2008 : • 86 live days / 4 months / 2x400g detectors • 18.3 kg.d with < 15 keV threshold, ~50% efficiency at 10keV • 2009 : physics run ongoing • 10 x 400g detectors running • x 20 improvement by 2010 : 4x10-8 pb • Additional new detectors with increased fiducial volume (« FIDs » 400g, 800g)  9 kg fiducial mass end 2010) Preliminary ID401+ID3 18.3 kgd TAUP09 - Roma - 07/03/2009 - SANGLARD Véronique

  14. The muon veto in action • Interactions in detectors due to muon-induced neutrons inside the shield • Geant4 • Expected : ~0.03 evts/kg.d < 250 keV • Mostly nuclear recoils below 50 keV • Measured detector - muon veto coincidence rate: ~ 0.04 evts/kg.d • The ionization yield distribution of coincidences is consistent with muon-induced events • In addition: neutron detector installed to study µ induced n Ionization/recoil energy 2007-2008 data (280 kg.d - no cut) TAUP09 - Roma - 07/03/2009 - SANGLARD Véronique

  15. Summary • Edelweiss-II now aiming at ~ 10-8 pb • Neutron shield and muon veto designed for < 10-8 pb • New generation of « ID » detectors are validated • Robust fabrication and operation • Thresholds <15 keV • Electron recoil rejection and surface event rejection (~1/105) • Good fiducial volume fraction - with improvments ongoing • ID detectors are well-fitted for future 100kg / ton-scale bolometric dark matter searches TAUP09 - Roma - 07/03/2009 - SANGLARD Véronique

  16. The future : EURECA • EURECA: beyond 10-9 pb, major efforts in background control and detector development • Joint effort from teams from EDELWEISS, CRESST, ROSEBUD, CERN, +others… • >>100 kg cryogenic experiment, multi-target : scintillators and Germanium • Part of ASPERA European Roadmap • Preferred site: 60 000 m2 extension of present LSM (4 /m2/d), to be dig in 2011-2012 • Design study start by the end of the year • Start installation by 2013 EURECA : 2 cryostats in water tanks TAUP09 - Roma - 07/03/2009 - SANGLARD Véronique

  17. Coincidences in µ-veto system and Ge bolometers 1keV<Edep<250keV Candidates for coincident veto-bolometer events Accidental coincidences Selection of coincident candidates: • > 1 veto module hit • 15 ms < Δt < 35 ms Geant 4 simulation of µ-induced Ge-hits Expected from Geant4 simulation: ~0.03 events/kg.d Measured: 0.04 events/kg.d TAUP09 - Roma - 07/03/2009 - SANGLARD Véronique

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