Searching for Dark Matter with the EDELWEISS Experiments

Searching for Dark Matter with 

the EDELWEISS Experiments 

Adam Cox 

Karlsruhe Institute for Technology (KIT) 

EDELWEISS Collaboration 

CEA, Saclay (IRFU and IRAMIS) IPNL (CNRS/IN2P3 and Université de Lyon) 

CSNSM Orsay (CNRS/IN2P3 and Université Paris-Sud) Neél Grenoble (CNRS/INP) 

KIT (IKP / EKP/ IPE), Karlsruhe JINR, Dubna University of Oxford University of Sheffield 

LSModane (CEA/CNRS)

• Briefly: Evidence for Dark Matter 

• Direct WIMP Dark Matter Detection Introduction 

• Edelweiss II 

• Experimental setup and results 

• Current Worldwide Direct Detection Status 

• Edelweiss III 

TU Dresden. G. Adam Cox 6 December 2012 

2

Astrophysical Evidence for Dark Matter 

• Galactic Rotation Curves - 

probably most compelling 


3


• Gravitational Lensing implies more matter than visibly accounted 


Bullet cluster 

4


• Cosmic Microwave Background model (measured by WMAP) 


5

What are we looking for in Edelweiss? 

• Gravitationally interacting 

• Weakly interacting with baryons, no electromagnetic interaction (no charge) 

• Long lived (~age of the Universe decay time) 


6

What are we looking for in Edelweiss? 

• Gravitationally interacting 

• Weakly interacting with baryons, no electromagnetic interaction (no charge) 

• Long lived (~age of the Universe decay time) 

• Neutrinos, Axions, KK particles (axinos, gravitinos, sneutrinos, ...) 

• cMSSM one possible Lightest Supersymmetric Particle (LSP): Neutralino is 

the most-popular WIMP candidate. Other LSPs exist depending upon the 

model and parameters. 

• cMSSM models give Neutralino M ~ 100 GeV and σA ~ 10 -9 pb (coherent 

elastic scattering) -- need large exposures to detect (~400 kgd in EDW 2) 


6

How to detect a WIMP 

• Weakly Interacting Massive Particle - searching for elastic/inelastic scattering of nuclei 

WIMP light 

heavy 

nuclei 


target 

material 

e - 

h + 

Na, I, Ar, Xe, Ge, Si, Ca, W, O... 

charge 

heat 

7

Direct Detection Experiments by Mechanism 

EDELWEISS 

CDMS 

SuperCDMS 

EURECA 

CoGeNT 



heat 

charge light 

XENON 

WARP, ArDM, 

LUX, ZEPLIN 

CRESST 

EURECA 

XMASS 

DAMA/LIBRA 

8

Ge Nuclei Recoil Signal Characteristics 

dR 

dE r 

= " 0 # 0 

2 

2m $m r 

F 2 (q) dv f νesc 

% 

& 1(v) 

v 

v min 

J.D. Lewin and P.F. Smith, Astropart. Phys. 6 (1996) 87 

(counts/keV/kg/yr/pb) 

-1 

 

dR 

dE 

! 

3 

10 

2 10 

10 

-1 

10 

-2 10 

0 2 4 6 8 10 12 14 16 18 20 

TU Dresden. G. Adam Cox 6 December 2012 Energy (keVnr) 

1 

f 1 = k " 

2 

v 

v0 3 e#v 2 / v 0 2 

WIMP Mass: 50 GeV 

Maxwellian 




vesc 

9


Include velocity of Earth 

dR/dE ~ 7% 


mχ = 30 GeV 

10


Include velocity of Earth 

dR/dE ~ 7% 


mχ = 8 GeV 

11

Edelweiss Signal Readout 



heat 

charge light 

12

Edelweiss Signal Readout 



heat 

charge light 

6 electrodes (e - , holes) 

NTD thermal sensor 

NTD 

12

EDELWEISS in a Nutshell 

• Array of cryogenically 

cooled Germanium 

crystals 

(10 in II, 40 in III) 

• Elastic Scattering of 

Ge by WIMPs 

• Rare event search: 

underground and 

shielded experiment 


13




crystals 



Ge by WIMPs 





Ge recoil produces heat + 

ionization 

Erecoil 

13




crystals 



Ge by WIMPs 





4800 mwe 

Ge recoil produces heat + 

ionization 

Polyethylene 

shield 

Muon Veto 

Pb shield 

cryostat 

Erecoil 

Neutron 

counter 

13

TU Dresden. G. Adam Cox 6 December 2012 14


Detection Principle 

Exploit Ionization Yield of Nuclear Recoils 

β, γ 


ELECTRON 

RECOIL 

15



β, γ 


ELECTRON 

RECOIL 

WIMP, NEUTRON 

NUCLEAR 

RECOIL 

~1/3 as many 

electron-hole pairs 

compared to 

electron recoil 

15



β, γ 

• Quenching or Ionization Yield - Q ~ 0.2 - 0.3 

• Distinguish nuclear recoil (NR) from electron recoil 

• Neutrons and WIMPs - nuclear recoil 


ELECTRON 

RECOIL 

WIMP, NEUTRON 

NUCLEAR 

RECOIL 

~1/3 as many 


compared to 


15



from AmBe calibration data 

16






NTD 

16


Erecoil ~ Eheat (after N-L corrections) 

Ionization Yield 

Eionization / Erecoil 





NTD 


(gammas) 

Ge nucleus recoil 

~1/3 as many 


16

Why Inter-Digitized Electrodes? 

Planar Electrode 

design in EDW 1 


17





Results from EDW 1 

17





Results from EDW 1 

Events near the 

surface suffer from 

charge collection loss 

17

Readout Channels 

charge carriers 

follow E-field lines 

+4.0 -1.0 

e- 

h 

-4.0 +1.0 

lattice excitations 

= thermal phonons 

TU Dresden. G. Adam Cox 6 December 2012 18 

NTD

CENPA MM. G. Adam Cox 17 September 2012 19

Event Location 

For each trigger, we have readout from all channels and use this to reconstruct the event. 

top view Ge crystal 


electrodes (e - , holes) 


20




Event Types: 




20




Event Types: 

Fiducial: A,C 




20




Event Types: 

Fiducial: A,C 

Surface: A,B 




20




Event Types: 

Fiducial: A,C 

Surface: A,B 




Surface Low Field: A,C +B 

20




Event Types: 

Fiducial: A,C 

Surface: A,B 





Pure Guard: G, H 

20




Event Types: 

Fiducial: A,C 

Surface: A,B 




n 


Pure Guard: G, H 

Other (anything but noise... usually multi-regional) 

20

ID Detectors: Surface Event Rejection 

Measurement 

• 210 Pb source to test for 

surface event rejection 

• Reject all events with signals 

on veto or guard electrodes 

• 6 x 10 -5 rejection factor 

• ID design state-of-the-art. To 

be used now in SuperCDMS 

(iZIP detectors) 


21

Putting it all together 

• Measure Heat and Ionization production from interaction 

• Quenching Factor distinguishes Nuclear Recoils from Electrons 

• Interleaved Electrode Design rejects surface betas 

• Next: Signal Processing Tools (plus calibration) 

• Next: Search for Fiducial Volume events in large data set 


22

Signal Processing Tools Needed 

• Bandpass filters / Wiener Optimal Filtering / Match Filters / Pulse 

Fitting / Cross-Talk Removal 

garde ID3AB baseline remove 

40 

20 

0 

-20 

-40 

0 1000 2000 3000 4000 

(Edw 2 pulses look different from Edw 3) 


garde ID3AB baseline remove + pattern remove 

20 

0 

-20 

0 1000 2000 3000 4000 

[10 μs] 

23

Signal Processing Tools Needed 

• Bandpass filters / Wiener Optimal Filtering / Match Filters / Pulse 

Fitting / Cross-Talk Removal 

~ 50 keV 


[1 second] 

Slow Heat pulse 

Not simple exponential 

Non-linearity 

24

Event Reconstruction: Erecoil and Q 

from Ionization and Heat Signals 

For each event, can 

express EI and EH as 


EI = QER 

EH = ER 

1+QV/✏ 

1+V/✏ 

25





rearranging 


EI = QER 

EH = ER 

Q = EI 

ER = 

ER 

✓ 

1+ V 

1+QV/✏ 

1+V/✏ 

✏ 

◆ 

EH 

V 

✏ EI 

25





rearranging 


EI = QER 

EH = ER 

Q = EI 

ER = 

ER 

✓ 

1+ V 

1+QV/✏ 

1+V/✏ 

✏ 

◆ 

EH 

Q γ ≡ 1 Q n = 0.165 ER 0.185 

V 

✏ EI 

25

Gamma Band and Nuclear Recoil Band 

neutrons from 

AmBe source 


133 Ba 

3+-1 10 -5 rejection of 

gammas in NR band 

26

Data Quality Cuts 

• Spin the Data and only accept events: 

• during stable cryogenic conditions 

• during stable low-noise environment 

• good Chi^2 cuts on pulse shapes 

• fiducial volume only events (of course) 

• reject time-correlated events with other detectors (neutrons) and the 

Muon Veto system (muons) 


27

“High” Energy Edelweiss 2 Results 

• July - Nov 2008 and April 2009 

- May 2010 

• 427 kg * days 

• 384 kg * days in 90% NR band 

• 5 events in NR band > 20 keV 


Phys. Lett. B 702 (2011) 329 

28

Direct Detection Status early 2012 


DAMA/LIBRA EPJ C56 (2008) 

CoGeNT PRL 106 (2011) 

CRESST II 2 sigma arXiv: 1109.0702 


CDMS Low E, PRL 106 (2011) 

CDMS + EDW, Phys Rev D 84 (2011) 

ZEPLIN III, arXiv: 1110.4769 

XENON100 PRL 107 (2011) 

XENON10 Low E, PRL 107 (2011) 

CMSSM Trotta et al, (2008) 

CMSSM with LHC and XENON 100 

Buchmueller et al, arXiv: 1110.3568 

29










XENON100 PRL 107 (2011) 





29










XENON100 PRL 107 (2011) 





29

Direct Detection Experiments 

• DAMA / LIBRA (light only): WIMPs? 

• XENON and ZEPLIN no WIMPs 

• CRESST WIMPs? 

• CoGeNT (charge only) WIMPs? 

• CDMS and EDELWEISS no WIMPs 


30








30








30







Energy spectrum 


Light yield distribution 

30







counts/30days 


30








30








30

Low Energy Analysis 

• Subset of detectors with exceptionally good resolutions and low backgrounds 

• Different parameterization of event observables (independent axis) 

• 5 - 20 keV analysis (independent data set) 

• Construct 2D (EI,ER) WIMP signal PDF, including detector / analysis efficiencies 

• Count number of events observed inside a 90% WIMP PDF region for a given 

mass and cross-section (no background subtraction or background model) 

• Find largest cross-section that corresponds to 90% CL Poisson limit given the 

observed number of events in the WIMP PDF region 


31

Low Energy Analysis 

Convert ONLY heat signal 

to nuclear recoil energy 

Independent: EI , ER 


32

Low Energy Data Set 

• 4 / 10 detectors used 

• 113 kg days 

• neutrons from calibration in gray 

• blue lines 95% gamma rejection 

• blue points - events that fall into 

the 90%CL region for a particular 

WIMP model used to test for upper 

limit cross section 


33

Setting Cross-Section Limits 

ID3, 10 GeV 


• For each WIMP mass 

• convolve all measured 

efficiencies for trigger, threshold, 

resolution, cuts, etc... 

• construct WIMP 2D PDF 

• determine 90%CL WIMP 

“Box” (red line in figure) 

• count # of events in box and set 

limit assuming Poisson statistics 

34

Setting Low Mass Limits 

ID2, 30 GeV 

ID6, 25 GeV 

3 candidates 


ID404, 20 GeV 

3 candidates 

ID6, 8 GeV 

No candidate 

35

Low Mass WIMP Cross Section Limits 


36

Maximum Likelihood Analysis for Low Mass 

Events / 30 days 

L = Y r⇢ (E,t) + 

16 

14 

12 

10 

8 

6 

4 

2 

0 

(1 r)⇢ (E,t) 

Events / 30 days 

16 

14 

12 

10 

0.2 0.4 

0 

0.6 0.2 0.8 0.4 1 0.6 0.8 

0 

0 1 

2 3 4 

0 

5 0 6 1 7 2 8 39 410 5 6 7 8 9 10 

Time [years] Time [years] 

Energy [keV] Energy [keV] 


8 

6 

4 

2 

Events / 0.67 keV 

16 

14 

12 

10 

8 

6 

4 

2 

Events / 0.67 keV 

mχ = 8 GeV 

σ ~7e-40 cm -2 

• Heat Energy / Time 

• Time-dependent model and 

efficiencies 

• ID3 

16 

14 

12 

10 

8 

6 

4 

2 

37

ection [cm 2 ] (normalised Cross−section to nucleon) [cm 2 ] (normalise 

40 

42 

44 

46 

48 

10 

Maximum Likelihood Analysis for Low Mass 

−42 

http://dmtools.brown.edu/ 

Spin Independent σA [cm -2 ] 

120712090201 

10 −44 

10 −40 

10-40 10 −46 

10 −42 

10-42 XENON1T (proj.) 

Preliminary 

10 −44 

10 

120712090201 −48 


Preliminary 

XENON1T (proj.) 

EURECA (proj.) 

10 50 

10 100 1000 

| | | | | | | | 

• Preliminary 

EDW-II PLB 702,5 (2011) 329 

DAMA/LIBRA http://dmtools.brown.edu/ 

EPJ C56 (2008) 


• In principle, CRESST should II 2σ arXiv:1109.0702 obtain better 

limits CRESST by using II entire 1σ arXiv:1109.0702 parameter 

space (should be relatively trivial 

CDMS Science 327, 1619 (2010) 

to add ionization energy) 

+ Low E, PRL 106 (2011) 

• Need XENON100 to have an arXiv:1207.5988 accurate 

gamma background model 

EURECA (proj.) 

+ PRD 86, 051701(R) (2012) 

EDW-III proj. 24kg(fid) 6 months 

EDW & CDMS comb. PRD 84 (2011) 

XENON100 PRL 107 (2011) 

Buchmüller et al, 2011 

Bertone et al, 2011 

• Improved analysis to come 

beginning of 2013 

WIMP Mass [GeV/c 2 10 100 1000 

WIMP Mass [GeV/c 

] 

2 10 50 

WIMP Mass [GeV/c] 2 ] 

38 

| | | |

EDELWEISS III 


EDW-3 projected 3000 kg.d 

24kg fid. mass x 125days 

39

Increased Exposure: More detectors and Fiducial Volume 

• Inter-Digitized --> Fully Inter-Digitized 

• Fiducial Volume: 40% --> 75% 

• 400g --> 800g 

• 40 New Detectors 

• 6 months = 3000 kg days 

• ~1 detector fabricated per week 

• Expect full delivery summer 2013 


FID800 

>600g 

40


Gammas 

• FIDs appear to show better 

gamma rejection (400k events) 

• Cleaner Tools: 

• Cabling 

• Holders 


42

Gammas 

• FIDs appear to show better 

gamma rejection (400k events) 

• Cleaner Tools: 

• Cabling 

• Holders 


42

Energy Resolution 

• New Front-End Electronics 

• No Feedback Resister on FET 

• Relays recharge electrodes 

• 500 eV FWHM (maybe 300 eV) 


EDW 2 

EDW 3 

43

Neutron Shielding 

• Redesigned cryostat 

• Block neutrons from Pb shield 

• Add inner PE shielding (in orange) 


Pb 

Pb Pb 

44

New Data Management / Routing System 

• Replace old, poorly functioning, unsupported hardware with new electronics 

from IPE group at KIT (also builds similar system for Auger, KATRIN) 


Single Crate 

Eliminates Many Hardware Issues 

Event Building 

Muon Veto + Other Subsystems 

Very Scalable (Next-Gen ready) 

Eventually will implement triggering 

on the crate 

45

Conclusions 

• Edelweiss II 

• Excellent background rejection with ID design 

• Setting limits at low-mass comparable to XENON/CDMS and in contradiction with 

CoGeNT / DAMA / CRESST 

• Maximum Likelihood (and other statistical methods) analysis to come 

• Edelweiss III 

• 32 kg of detector mass (24 kg fiducial) starting mid-2013 

• Less background, better energy resolution 

• Strong limit (or discovery) for 2013/2014 


46

Searching for Dark Matter with the EDELWEISS Experiments

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