From Edelweiss I to Edelweiss II Véronique SANGLARD CNRS/IN2P3/IPNL sanglard@ipnl.in2p3.fr http://edelweiss.in2p3.fr

Outline The Edelweiss-I experiment 2003 results Ionization trigger data taking Phonon trigger data taking The second phase : Edelweiss-II Perspectives and conclusion

The Edelweiss* collaboration CEA-Saclay DAPNIA/DRECAM CRTBT Grenoble CSNSM Orsay FZK/Univ. Karlsruhe IAP Paris IPN Lyon Laboratoire Souterrain de Modane 1700 m depth under the Fréjus tunnel (4800 we) 4 µ/m²/d (106 less than at the surface) *Expérience pour DEtecter Les WIMPs En SIte Souterrain (Underground experiment to detect WIMP)

Heat and ionization detectors threshold Simultaneous measurement of charge and heat signals for each interaction Different charge/heat ratio for nuclear and electron recoils (γs, βs ionize more than WIMPs and neutrons) Neutrons 73Ge(n,n',γ) Gammas Event by event discrimination Discrimination > 99.9 % for Erec>15 keV

Charge collection Miscollected charge events can simulate nuclear recoils Use of gamma calibration (57Co, 137Cs) to check the detector charge collection quality Few miscollected charge events with amorphous layer Since 2002 use detectors with amorphous layer WITH WITHOUT

The 1 kg stage Shield : 30 cm paraffin 15 cm lead 10 cm copper Al sputtered electrodes NTD heat sensor Ge or Si amorphous layer Guard ring fiducial vol. : 57 %* *(O.Martineau et al. Nim A in press)

2003 Edelweiss data Additional ~45 kg.d recorded with 3 new detectors 2 phases with 2 different triggers On the ionization signal On the phonon signal 2000-2003 data represent ~ 62 kg.d Results : Events observed in nuclear recoil band (40 for Erec > 15 keV)

2003 data (ionization trigger) 20 kg.d Energy threshold : 20, 30 keV 3 events observed in the nuclear recoil band (above these thresholds)

2003 data (phonon trigger) Lower energy threshold : 15 keV 18 events observed in nuclear recoil band, most (12) below 30 keV in 22 kg.d 1 coincidence n-n observed between detectors (10% prob.) Stable behavior of 3 detectors over total exposition

Edelweiss new limit PRELIMINARY Unknown backgrounds "Yellin method"* used to derive exclusion limit *(PRD 66,032005 (2002)) No background subtraction New (prel.) limit consistent with the previous publication* *(Phys. Lett. B 545 43 (2002)) PRELIMINARY

Experimental spectrum Low energy spectrum inconsistent with Wimp mass > ~ 20 GeV Possible backgrounds Neutrons (n-n coinc.) Miscollected charge events (surface events)

Lessons from Edelweiss-I With 3 new detectors and an extended exposure, the preliminary 2003 exclusion limit confirms the previous published one Surface events : Improved radiopurity in Edelweiss-II Identification (or suppression) possible with NbSi thin film sensor Neutron background : Improved shielding against neutron Anti-coincidences more efficient with increased number of detectors

Identification of surface events 2 NbSi athermal phonon sensors for surface event rejection Two components : Thermal (energy) Athermal/transitory (near-surface tag) For this surface event, the athermal component is higher in NbSi 1 First tests of 200g modules in Edelweiss-I promising : 10 x less background while retaining 50 % efficiency

Perspectives : Edelweiss-II Aim : x 100 improvement in sensitivity 1st phase : 21*320g Ge bolometers with NTD heat sensor 7*400g Ge bolometers with NbSi thin film sensor Installation started in April 2004 Data taking in 2005

Edelweiss II : new cryostat Larger experimental volume Low radioactivity cryostat Innovative reversed geometry 10 mK base temperature First phase : 28 detectors, up to 120

Edelweiss II : new setup Clean room Efficient shielding against neutron and gamma ray background 20 cm lead 50 cm PE Muon veto Sensitivity Edelweiss I : 0.2 evt/kg/day Edelweiss II : 0.002 evt/kg/day

Conclusion 2003 preliminary data confirm the 2002 exclusion limit Edelweiss-I is sensitive to -optimistic- SUSY models (10-6 pb) Edelweiss-II, goals : To reach more favored SUSY models (10-8 pb) Competitive with CDMS-II, CRESST-II Testing the bulk of SUSY parameter space (>10-10 pb) will require one-ton detector array and an extreme background rejection

The Modane Underground Laboratory Edelweiss I Edelweiss II 1700 m depth under the Fréjus Tunnel (4800 we) 4 µ/m²/day (106 less than at the surface) 1500 neutrons (>1 MeV)/m²/day (rock radioactivity)

Direct detection Principle : Elastic scattering of a WIMP on a target nucleus Constraints : Low energy deposited : Erecoil <1OO keV Small interaction rate : R<1 evt/kg/day Electronic recoil : background Nuclear recoil : signal

Neutron-gamma discrimination Use of neutron calibration to simulate WIMP nuclear recoils Evt/evt Discrimination (between nuclear and electronic recoils) > 99.9 % for Erec > 15 keV Use n-n coincidences to identify neutron background Ionization Threshold Neutrons 73Ge(n,n',γ) Gammas

Edelweiss-I 2002 data 20

Resolutions

Phonon trigger data Trigger efficiency very precisely determined by coincidence spectrum in neutron data Threshold at 50 % efficiency (in recoil energy)

Expected sensitivity

137Cs calibration with new trigger (1) High statistics γ 137Cs calibration (~105 events) Charge collection quality (check of γ rejection) Exposure (in gamma rays) corresponding to ~ 2 years of data taking ~ 10 nuclear recoil events observed after one week ~ 31 after two weeks Coincidences (between nuclear recoils) observed between detectors

137Cs calibration with new trigger (2) Top detector 5 events in nuclear recoil band with 1 coincidence Further away from background pollution Middle detector 11 events in nuclear recoil band with 2 coincidences Closer to background pollution Bottom detector 15 events in nuclear recoil band with 1 coincidence Closest to background pollution

137Cs calibration with new trigger (3) Summary of all three detectors ~ 22 (Erec > 15 keV) events in nuclear recoil band > 50 000 events below 200 keV recoil energy

137Cs calibration with new trigger (4) What are really these events ? Compatible with nuclear recoils Some of them are coincidences between detectors Probably neutrons 137Cs calibration suggests a contamination by 252Cf source Confirmed by measurement of the source holder in low-background Ge diode setup Edelweiss sensitive to low nuclear recoil rates