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

Direct detection constraints• Spherical dark matter halo

• Elastic scattering on target nucleusa few 10s keV deposited

• Rare events (< 1 evts/kg/day)Low background environment: • Underground laboratory

• Passive and active shielding

• Low radioactivity Materials

High target mass & Long time run (>year)

• Experimental signatures:• Annual or daily modulation

• Comparison of several absorbers• Active discrimination of radioactive background

Edelweiss detectors environment

WIMP

HeatHeat

IonizationIonization

LightLight

≈ few % energy

≈ 20 % energy

≈ 100% energycryogenic detectors

absorberabsorber

GeGe

NaI, XeNaI, Xe

Al2O3,LiFAl2O3,LiF

DAMA (Italie)

IGEX(US/Russ) HDMS(Ger/Russ)

Liquid XeLiquid Xe

Ge, SiGe, Si

CaWO4, BGOCaWO4, BGO

EDELWEISS (Fr/Ger) CDMS (US)

CRESST(Ger) Rosebud(Spa/Fr)

ZEPLIN (GB) XENON (US) XMASS (Jap)

Detection methods

Heat and ionisation detectors

Wimp

Scattered

Wim

p

NTD sensor

electrons

holes

heat channel

« Centre » Ionisation channel

• IonisationIonisation: some thousands pairs over some 100ns • 2 Al sputtered electrodes (Centre+Guard)

• HeatHeat: some µK over ms

• Neutron Transmutation Doped thermistor

320g high purity Ge detectors

Ge

« Guard » Ionisation channel

E

Event by event discriminationNeutron source calibration

73Ge(n,n’,)

Ionisation theshold

Neutrons, WIMPsNuclear recoils

• Different heat/charge ratio for electron and nuclear recoil

• Discrimination>99.9% for Erecoil>15keV

Gammas, electrons

Electron recoils

Edelweiss-I 1kg stageWhat’s new ? Energy threshold improvement

• previous 2003 results:previous 2003 results: 3320g detectors, additional 20kg.day fiducial exposure with ionisation trigger (100% efficiency for Erec>30keV)

• latest results (preliminary):latest results (preliminary): additional 22.66kg.day fiducial exposure with phonon trigger (100% efficiency for Erec>15keV)

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

• Sensitivity confirmed with 61.8 kg.d total exposure

• DAMA candidate excluded at 99.8% CL for Mwimp44GeV

• Model independent exclusionCopy & Krauss, Phys.Rev.D67, 2003Kurylov & Kamionkowski, phys.Rev.D69, 2004

DAMA candidate

EDELWEISS-I last results: astro-ph/0503265

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

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

• Low 73Ge content balanced by nuclear recoils discrimination and high neutron nuclear spin

• Indirect detection still 10 times more sensitive (Baksan, Super-K)

Sensitivity of EDELWEISS to spin-dependent interactions: astro-ph/0412061

EDELWEISS-II• 100 liter cryostat for up to 120 detectors : ≈ 36 kg Ge100 liter cryostat for up to 120 detectors : ≈ 36 kg Ge• Improve sensitivity by factor ~100Improve sensitivity by factor ~100

• EDELWEISS I: ~0.2evt/kg/day (~10-6pb)• EDELWEISS II: ~0.002evt/kg/day (~10-8pb)

• Assembly in progressAssembly in progress• First data taking: september 2005First data taking: september 2005

Reversed cryostat, base Temperature: 10mK Close packed detectors

(hexagonal arrangement)

EDELWEISS-IIimproved background rejection

EDELWEISS-IEDELWEISS-I 2 main limitations:2 main limitations:

• neutron background

• Miscollected near-electrode events

EDELWEISS-IIEDELWEISS-II 2 main improvements:2 main improvements:

• muon veto + improved shielding: 20cm lead, 50cm PE

• near-electrode events identification

Near-electrode events identificationwith NbSi bolometers

NbSi 1

NbSi 2

Near electrode event

• 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

200

150

100

50

0

806040200

x10-3

200

150

100

50

0

806040200

x10-3

Transitory thermal

Time (a.u.)

Heat signal NbSi 1NbSi 2

Mirabolfathi et al., 2001

Near-electrode events identificationwith NbSi bolometers

• Efficient method down to threshold energy• Qualification in Modane: rejection:25% of events, 83% of low-Q events• Further improvements: better energy resolution, reproducibility• 7 NbSi bolometers in EDELWEISS-II first phase

57Co calibration run Same run, cut on transient pulse

Er(keV) Er(keV)

Q=

Ei/

Er

Near-electrode events identificationwith ionisation channel

10

8

6

4

2

0

Sign

al (

mV

)

-800 -400 0 400 800Time (ns)

event 122keV

Experimental signal

Best fit by simulation

Electrons collected

Holes collected

Indu

ced

char

ge(A

.U)

Time (ns)Broniatowski et al., 2001

• Time-resolved ionisation measurements + carrier transport simulation code allows position identification• 1mm resolution @122keV on test detectors• Further improvement: High Electron Mobility Transistor at 4K

Other applications:

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

Conclusion

• Say good bye to EDELWEISS-I Say good bye to EDELWEISS-I

• Understanding the backgroundUnderstanding the background

• R&D work on detectorsR&D work on detectors

• European (Eureca) and american European (Eureca) and american (Super CDMS) projects for 1 ton (Super CDMS) projects for 1 ton targettarget

CDMS-II, CRESST-II, EDELWEISS-II,XENON, XMASS sensitivity goals(~a few events/ton/day)

1 Ton sensitivity goal (optimistic)(~a few events/ton/year)

CDMS, CRESSTEDELWEISS-I present(~0.1 event/kg/day)

L. Rozkowski et al., hep-ph/0208069