EDELWEISS-I last results EDELWEISS-II prospects for dark matter direct detection

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EDELWEISS-I last results EDELWEISS-II prospects
for dark matter direct detection
CEA-Saclay DAPNIA and DRECAMCRTBT GrenobleCSNSM
OrsayIAP ParisIPN LyonModane Underground
Laboratory (Fréjus) FZ-Karlsruhe and Univ.
Karlsruhe
B. Censier, CSNSM Orsay, France for the EDELWEISS
collaboration
Astroparticle Montpellier Toulouse meeting -
29/04/2005
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Direct detection constraints

• Spherical dark matter halo
• Elastic scattering on target nucleus
• a few 10s keV deposited
• Rare events (lt 1 evts/kg/day)
• Low background environment
• Underground laboratory
• Passive and active shielding
• Low radioactivity Materials
• High target mass
• Long time run (gtyear)
• Experimental signatures
• Annual or daily modulation
• Comparison of several absorbers
• Active discrimination of radioactive background

Edelweiss detectors environment
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Detection methods
WIMP
20  energy
Ionization
absorber
Heat
 100 energycryogenic detectors
Light
few  energy
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Heat and ionisation detectors
320g high purity Ge detectors
 Centre  Ionisation channel
 Guard  Ionisation channel
electrons
Ge
E
heat channel
Wimp
NTD sensor
holes
Scattered Wimp

• Ionisation some thousands pairs over some 100ns
  
• 2 Al sputtered electrodes (CentreGuard)

• Heat some µK over ms
• Neutron Transmutation Doped thermistor

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Event by event discrimination Neutron source
calibration
73Ge(n,n,?)
Ionisation theshold

• Different heat/charge ratio for electron and
  nuclear recoil
• Discriminationgt99.9 for Erecoilgt15keV

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Edelweiss-I 1kg stageWhats new ? Energy
threshold improvement

• previous 2003 results 3?320g detectors,
  additional 20kg.day fiducial exposure with
  ionisation trigger (100 efficiency for
  Erecgt30keV)
• latest results (preliminary) additional
  22.66kg.day fiducial exposure with
  phonon trigger (100 efficiency for Erecgt15keV)

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Wimp-nucleon cross-section constraints
(Spin-independent)

• Sensitivity confirmed with 61.8 kg.d total
  exposure
• DAMA candidate excluded at 99.8 CL for
  Mwimp?44GeV
• Model independent exclusion
• Copy Krauss, Phys.Rev.D67, 2003
• Kurylov Kamionkowski, phys.Rev.D69,
• 2004

DAMA candidate
EDELWEISS-I last results astro-ph/0503265
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Wimp-nucleon cross-section constraints
(Spin-dependent)

• Two types of coupling to matter to be
  considered
• Scalar coupling (spin-independent) ? (mass
  number)2 ? dominant for heavy nuclei
• Axial-vector coupling (spin-dependent) ? nuclear
  spin (from unpaired proton or neutron)
• 7.8 of natural Ge is 73Ge, a high-spin isotope
• 4.8kg.day of exposure for spin-independent
  interaction
• Even with high-spin nuclei, direct detection
  sensitivity is orders of magnitude lower for
  spin-dependent

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Wimp-nucleon cross-section constraints
(Spin-dependent)
Sensitivity of EDELWEISS to spin-dependent
interactions astro-ph/0412061

• Low 73Ge content balanced by nuclear recoils
  discrimination and high neutron nuclear
  spin
• Indirect detection still 10 times more sensitive
  (Baksan, Super-K)

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EDELWEISS-II

• 100 liter cryostat for up to 120 detectors
   36 kg Ge
• Improve sensitivity by factor 100
• EDELWEISS I 0.2evt/kg/day (?10-6pb)
• EDELWEISS II 0.002evt/kg/day (?10-8pb)
• Assembly in progress
• First data taking september 2005

Reversed cryostat, base Temperature 10mK
Close packed detectors (hexagonal arrangement)
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EDELWEISS-IIimproved background rejection

• EDELWEISS-I
• 2 main limitations
• neutron background
• Miscollected near-electrode events

• EDELWEISS-II
• 2 main improvements
• muon veto improved shielding 20cm lead, 50cm
  PE
• near-electrode events identification

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Near-electrode events identificationwith NbSi
bolometers

• Thin evaporated NbSi layers near metal/insulator
  transition developped at CSNSM Orsay
• Good coupling with Ge absorber allows
  out-of-equilibrium phonons detection
• Simultaneous charge measurement by Nb electrodes
• Near-electrodes events have an enhanced
  transitory part

NbSi 1
Near electrode event
NbSi 2
Mirabolfathi et al., 2001
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Near-electrode events identificationwith NbSi
bolometers
57Co calibration run
Same run, cut on transient pulse
QEi/Er
Er(keV)
Er(keV)

• Efficient method down to threshold energy
• Qualification in Modane rejection25 of
  events, 83 of low-Q events
• Further improvements better energy resolution,
  reproducibility
• 7 NbSi bolometers in EDELWEISS-II first phase

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Near-electrode events identificationwith
ionisation channel

• Time-resolved ionisation measurements carrier
  transport simulation code allows position
  identification
• 1mm resolution _at_122keV on test detectors
• Further improvement High Electron
  Mobility Transistor at 4K

? event 122keV
Experimental signal
Holes collected
Best fit by simulation
Induced charge(A.U)
Electrons collected
Time (ns)
Broniatowski et al., 2001

• Other applications
• Double-beta decay
• Studies on Electronic transport, space-charge,
  quality of charge collection

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Conclusion

• Say good bye to EDELWEISS-I
• Understanding the background
• RD work on detectors
• European (Eureca) and american (Super CDMS)
  projects for 1 ton target