Status of the Cryogenic Dark Matter Search

(CDMS) Experiment

Bruno SerfassUniversity of California, Berkeley

for the CDMS Collaboration

Rencontres de Moriond – March 2005

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Stanford University

P.L. Brink, B. Cabrera, J.P. Castle,C.L. Chang, M. Kurylowicz, L. Novak, R. W. Ogburn, T. Saab, A. Tomada

University of California, Berkeley

J. Alvaro-Dean, M.S. Armel, M. Daal, J. Filippini, A. Lu, V. Mandic, P.Meunier, N. Mirabolfathi, M.C.Perillo Isaac, W. Rau, B. Sadoulet, D.N.Seitz, B. Serfass, G. Smith, A. Spadafora, K. Sundqvist

University of California, Santa Barbara

R. Bunker, D.O. Caldwell, D. Callahan, R.Ferril, D. Hale, S. Kyre, R. Mahapatra, J.May, H. Nelson, R. Nelson, J. Sander, C.Savage, S.Yellin

University of FloridaL. Baudis, S. Leclercq

University of Minnesota

J. Beaty, P. Cushman, L. Duong, A. Reisetter

Brown University

M.J. Attisha, R.J. Gaitskell, J-P. F. Thompson

Case Western Reserve University

D.S. Akerib, M.R. Dragowsky, D.D.Driscoll, S.Kamat, A.G. Manalaysay, T.A. Perera, R.W.Schnee, G.Wang

University of Colorado at Denver

M. E. Huber

Fermi National Accelerator Laboratory

D.A. Bauer, R. Choate, M.B. Crisler, R. Dixon, M. Haldeman, D. Holmgren, B. Johnson, W.Johnson, M. Kozlovsky, D. Kubik, L. Kula, B. Lambin, B. Merkel, S. Morrison, S. Orr, E.Ramberg, R.L. Schmitt, J. Williams

Lawrence Berkeley National Laboratory

J.H Emes, R. McDonald, R.R. Ross, A. Smith

Santa Clara University

B.A. Young

CDMS II: The People…

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Cryogenic Dark Matter Search (CDMS) Experiment designed to search for Dark Matter in the form of WIMPs

Detect them via elastic scattering on nuclei (nuclear recoils). Dominant backgrounds are electromagnetic in origin (electron recoils)

WIMPs: Extremely small scattering rate (fraction of 1 evt/kg/day), small energy of the recoiling nucleus (falling exponentially with E ~ 15 keV)… Distinguish electron recoils (gammas, betas) from nuclear recoils (neutrons, WIMPs) event by event using Ge (Si) based detectors with two- fold interaction signature:

Ionization signal Athermal phonon signal

Suppress neutron background by:

Going deep underground

Soudan mine: 713m below the surface

Active muon veto, polyethylene shielding Relative event rates: Singles vs multiples, Ge vs Si

CDMS II Overview

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Ionization Yield EQ/ER

Y~ 1 for electron recoils

Nuclear Recoils (252Cf)

Nuclear Recoils (252Cf)

WIMPS (and neutrons) scatter off nuclei

Identify nuclear recoils event by event!

Y~ 0.3 (Ge) for nuclear recoils

• Events occuring near the surface (<~10 m) have an incomplete charge collection (“dead layer”) and can be misidentified as nuclear recoils

Nuclear Recoils (252Cf)

Nuclear Recoils (252Cf)

• Surface events:

Electrons produced by radioactive beta decays from surface contamination

Electrons ejected from nearby material by high energy x-rays

Gammas interacting within ~10 m of the surface

Most background sources (electrons, photons) scatter off electrons

Measure simultaneously ionization and athermal phonons

CDMS II Overview

Bulk Electron Recoils (133Ba)

Bulk Electron Recoils (133Ba)

51 tungsten380 x 60 aluminum fins

Q inner

Q outer

A

B

D

C

Rbias

I bias

SQUID array Phonon D

Rfeedback

Vqbias

The ZIP Ionization & Phonon Detectors

Qouter

Qinner

zy

x

Measure ionization in low-field (~volts/cm) with segmented contacts to allow rejection of events near outer edge

250 g Ge or 100 g Si crystal 1 cm thick x 7.5 cm diameter Photolithographic patterning Phonon sensors:

• 4 quadrants with each 888 sensors (TES) operated in parallel

• TES: 1-m-thick strip of W connected to 8 superconducting Al collection fins

2 charge electrodes:

• “Inner” fiducial electrode• “Outer” guard ring

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The ZIP Towers

Ge (Z3)

Si (Z4)

Ge (Z5)

Si (Z6)

Ge (Z1)

Ge (Z2)

4 K

0.6 K

0.06 K

0.02 K

FET cards

Tower 1:

• 4 Ge and 2 Si ZIPs

• Thoroughly understood at Stanford

• Beta background on bottom Si detector (Z6)

SQUID

5 Towers now installed! 30 detectors: 19 Ge (4.75 kg) and 11 Si (1.1 kg)

Tower 2: 2 Ge and 4 Si

Tower 3, 4: 4 Ge and 2 Si each

Tower 5: 5 Ge and 1 Si

Tower 1

And…

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Detector response 4 phonon pulses 2 charge pulses (Qinner, Qouter)

Informations on phonons pulse shape (ex. risetime), delay between charge and phonon pulses

Phonon sensors provide measurement of xy position:

• Phonons propagate at 0.5 (1) cm/s in Ge (Si) crystal measurable delays between the pulses of the 4 phonon channels

• Able to measure x,y coordinates of interaction

• Demonstrate by shining sources through a collimator

Cd109 + Al foil: 22 kev

Delay Plot

We can correct the phonon energy/timing position dependence

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Z-Position Sensitivity Rejects Surface events

Energy deposited near the surface gives rise to slightly lower-frequency phonons

undergo less scattering and hence travel ballistically

Shorter risetime than bulk events

Bulk eventSurface event

Surface event:

Overall rejection of surface events appears >99%

We are only beginning to take full advantage of the information from the athermal phonon sensors!

Improving modeling of phonon physics Extracting better discrimination parameters (timing and energy partition)

Neutrons

Gammas

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CDMS II at Stanford and at Soudan Log

10(M

uon

Flu

x)

(m-2s

-1) Stanford Underground

Facility (SUF)

Depth (meters water equivalent)

500 Hz muons in 4 m2 shield

2001-2002 run at Stanford (17 mwe of rock) 28 kg-day exposure of 4x 250g Ge detectors (and 2x 100g Si detectors)

20 nuclear-recoil candidates consistent with expected neutron background PRD 68:082002 (2003)

Soudan Mine1 per minute in 4 m2 shield

2003-2005 in Soudan Mine (Minnesota)

Depth 713 m (2090 mwe)

Reduce neutron background:

~1/kg/day to ~ 1/kg/year

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Shielding, Veto at Soudan

Layered shielding (reduce , , neutrons) 40 cm outer polyethylene

Removes neutrons from (,n) 22.5 cm Pb, inner 5 cm is “ancient” 10 cm inner polyethylene

Removes neutrons from muons ~0.5 cm Copper walls of cold volume

Active Muon Veto

Hermetic, 2” thick plastic scintillator veto wrapped around shield

Reject residual cosmic-ray induced events

Veto rate ~600Hz

One muon per minute is incident on the veto

Lead Polyethylene

mu-metal (with copper inside)

Ancient lead

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Summary of data taking at Soudan

Oct. 2003 - Jan. 2004: Run (118) of Tower 1

4 Ge (1 kg) and 2 Si (0.2 kg) ZIPs (same tower as run 21 at Stanford)

53 live-days after in 92 calendar days

Efficiency nearly

85% for last six weeks

Gaps were cryogenic fills and calibration runs (133Ba, 252Cf)

Mar. 2004 – Aug. 2004: Run (119) Towers 1,2

12 detectors: 6 Ge (1.5 kg) and 6 Si (0.6 kg)

~70 live-days after in 137 calendar days

Soon beginning: Run (120) Towers 1-5

30 detectors: 19 Ge (4.75 kg) and 11 Si (1.1 kg)

Run 118

Run 119

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Run 118 (Tower 1): Energy calibration with 133Ba source

Use 356 keV 133Ba lines to calibrate Ionization

10.4 keV (Ge activation), 303 keV, and 384 keV lines confirm linearity

Calibrate Si using Monte Carlo

Phonons calibrated to charge

Good agreement with the simulations

Ionization energy in keVIonization energy in keV

Phonon energy in keVPhonon energy in keV

MCdata

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Cuts and Efficiency for Nuclear Recoils

No veto hit (97%) nor bad noise pre-trigger (95%)

Ionization yield (< 95%)

Timing cuts (vary with energy from ~30% to 80%)

ionization threshold In fiducial volume (< 85%)

Data cuts and threshold based on in situ gamma and neutron calibration

Blind analysis: The WIMP-search data were in “sealed box” (in particular nuclear-recoil region) until cuts finalized

Z1 threshold at 20 keVZ2, Z3, Z5 thresholds at 10 keV

Run 118

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WIMPs search data with Ge detectors (Run118)

Yellow points from neutron calibration

Ch

arg

e Y

ield

Prior to timing cuts After timing cuts

Blue points from WIMP search data (Z2, Z3, Z5)

Recoil energy (keV) Recoil energy (keV)

Ch

arg

e Y

ield

Expected background: 0.7 ± 0.35 mis-identified surface electron recoils ~0.07 unvetoed neutrons

Event

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CDMS limit from Soudan

Exposure after cuts of 52.6 kg-d raw exposure with Ge ≈ 20 kg-days for recoil energies between 10-100 keV

No nuclear-recoil candidates (1 candidate with non-blind analysis)

Expect ~0.7 mis-identified surface electron recoils, ~0.07 unvetoed neutrons (1.0 muon coincident neutron)

New limit ~10x (x4) better than CDMS SUF (EDELWEISS) at a WIMP mass of 60 GeV/c2

Hard to accommodate DAMA annual modulation effect as a WIMP signal!

DAMA

CDMS SUF

EDELWEISS

CDMS Soudan

Minimum of the limit curve: 4 x10-43 cm2 at 90% C.L for a WIMP mass of 60 GeV/c2

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• Soudan Tower 1-2 (R119) in 2004 Analysis well underway plan to announce results at April APS!

Expected Run119

• Soudan Towers 1-5 (R120) in 2005

5 towers installed (19 Ge and 11 Si detectors) Cryogenic, electronics, DAQ upgrades

Expected

Run120

What’s next for CDMS II?

DAMA

CDMS-II explores MSSMs in series of runs:

• SUF Tower 1 in 2002

• Soudan Tower 1 (R118) in 2003/04 PRL 93, 211201 (2004) More details PRD submission in March

Current CDMS limit

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Toward a ton-scale experiment: SuperCDMS

Remove Muon-induced Neutron Background…

…. by moving further down

At Stanford: 17 mwe, 0.5 n/d/kg At Soudan: 2090 mwe, 0.5 n/y/kg At SNOLab: 6060 mwe, 1 n/y/ton

Stanford

Soudan

Sudbury (Canada)

Worry about neutrons from residual

radioactivity only

Reduce photon and electron backgrounds

Improve analysis, phonon-timing cuts Reduce raw rates via better shielding, cleanliness Improve detectors: Increase detector thickness, double-sided phonon sensors, interleaved ionization electrodes

Sensitivity improve:• If no background: Linearly with M (detector mass) and T (exposure time)• If background that can be estimate independently: √MT

Increase mass, remove backgrounds

SuperCDMS: Scientific goals

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Conclusion

The CDMS II experiment at the Soudan mine is at the forefront of the field.

2 runs are completed: Run of Tower 1 (53 livedays with 4 Ge and 2 Si detectors)

• Results incompatible with DAMA for standard halo and WIMPS, PRL 93, 211201 (2004)

Run of Towers 1 and 2 (~70 livedays with 6 Ge and 6 Si detectors)

• Analysis well underway, results to be announced in April 2005

5 towers now installed (19 Ge and 11 Si detectors)

Development project toward a ton scale: SuperCDMS

Zero-background goal

Sensitivity to study WIMP physics down to ~10-46 cm2

Submitted Development Project proposals

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Backup slides

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Electrothermal Feedback

Voltage bias supplied Joule heating P = V2/R

Quasi particles heat up W T R P P T Stability

ElectroThermal Feedback

R

T

Rshunt

Ibias

W ETF-TES

SQUID Array Measure reduction in

Joule heating by change in current

I = V/R E = IV dt

Nuclear recoils in Ge ZIPNuclear recoils in Ge ZIP

Cou

nts/

(ke

V k

g da

y)

Recoil Energy (keV)

103

102

Expectations from simulation

Data

Nuclear recoils in Si ZIPNuclear recoils in Si ZIP

Expectations from simulation

Recoil Energy (keV)

104

103

102

Cou

nts/

(ke

V k

g da

y) Data

Phonon calibration does not depend on whether the event is a nuclear recoil or electron recoil

0

5

10

15

0

2

4

6

8

10-1

-0.5

0

0.5

1

X Position [mm]Z Position [mm]

Volta

ge [V

]Interleaved Ionization electrodes concept

Alternative method to identify near-surface events Phonon sensors on both sides are virtual ground reference. Bias rails at +3 V connected to one Qamp Bias rails at -3 V connected to other Qamp Signals coincident in both Qamps correspond to events drifted out

of the bulk. Events only seen by one Qamp are < 1.0 mm of the surface.

Double-sided phonon sensors

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Theory survey - earlier MSSM

Baltz & Gondolo PRD67 065503 (2003)Kim,Nihei,Roszkowski, hep-ph/0208069

Baltz & Gondolo hep-ph/0102147

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Constrained MSSM and relax GUTs

Baer et al, hep-ph/0305191Chattopadhyay et. al, hep-ph/0407039Ellis et al, hep-ph/0306219

Bottino, et al hep-ph/0307303

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mSUGRA and Split Supersymmetry

Baltz & Gondolo hep-ph/0407039 A. Pierce, hep-ph/0406144 &G. F. Giudice and A. Romaninohep-ph/0406088

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NeutronProton

Spin dependent WIMP-nucleon Interactions

Preliminary