First Results from the Cryogenic Dark Matter Search at the Soudan Underground Laboratory

ICHEP Beijing, ChinaAugust 16-22, 2004

Professor Priscilla CushmanUniversity of Minnesota

First Results from the Cryogenic Dark Matter Search at the Soudan

Underground Laboratory

Priscilla CushmanUniversity of Minnesota



Composition of the Cosmos

WIMPsWMAP best fit



Moving from a Shallow Site at Stanford to a Deeper Site at the Soudan Underground Mine in Northern Minnesota

1 per minute in 4 m2 shield

Depth (mwe)

Log 1

0(Muo

n Fl

ux) (

m-2s-1

)

Stanford Underground Site500 Hz muons in 4 m2 shield

Depth 713 m (2090 mwe)Reduce neutron background from ~1 / kg / day to ~1 / kg / yearReduce cosmic muon flux by ~ 30,000



Brown University M. Attisha, R.J. Gaitskell, J.P. Thomson, Case Western Reserve UniversityD.S. Akerib, M. Dragowski, S. Kamat, R.W. Schnee, G.WangFermi National Accelerator LaboratoryD. Bauer, M.B. Crisler, D. Holmgren, E. RambergLawrence Berkeley National LaboratoryJ.H. Emes, A. SmithUniversity of Florida

L. BaudisSanta Clara UniversityB.A. Young

University of Minnesota, MinneapolisL. Duong, P. Cushman, A. ReisetterStanford UniversityP.L. Brink, B. Cabrera, C.Chang,R.W. Ogburn

University of California, BerkeleyV. Mandic, P. Meunier, N. Mirobalfathi, B. Sadoulet, D. Seitz, B. Serfass, K. SundqujstUniversity of California, Santa BarbaraR. Bunker, D. O. Caldwell, R. Mahapatra, H. Nelson, J. Sander, S. Yellin.

University of Colorado at DenverM. E. Huber

The CDMS Collaboration

…in the mine



The Soudan Underground Laboratory

MINOS

CDMS II

Old: Soudan2 proton decay calorimeterNew: Screening and Prototyping Area Applications welcome, see

http://www.hep.umn.edu/~prisca/soudan

Operated by the University of Minnesota, in cooperation with Fermi National Accelerator Laboratory

and the Minnesota Department of Natural Resources



Dan BauerNSF/DOEOct. 8, 2002Soudan Installation

¥ CDMS II Experimental Enclosures ( Fermilab, Minnesota)

H V A C

M e c h a n i c a l

R F - s h i e ld e dC lean room

Shield

F ridge

F ro n t-e n dElectronics

Mezzanine Mezzanine

D etecto r P rep

D A Q /E l e c tro n i c s

Clean BenchesIceboxP um ps,C ryo gen ics

Soud

an

2

Mino

s en

tranc

e

Dan BauerNSF/DOEOct. 8, 2002

CDMS II Facilities

Mezzanine

Main floor

Electronics roomClean Room Loft

Offices

MINOS staging

Detector Prep

Cleanroom

Main floor

Mezzanine



CDMS Icebox and Shield

plasticscintillators

polyethylene

lead

ancientlead inner

polyethylene

Dilution Fridge

|------------------ 2.18 m --------------------------|



Q inner

Q outer

A

B

D

C

Rbias

I bias

SQUID array Phonon D

Rfeedback

Vqbias

ZIP Detectors and Tower Construction

4 Phonon Channels: XY position Z position using timing and signal shape 2 Charge Channels: radial position

250 g Ge or 100 g Si crystal1 cm thick x 7.5 cm diameter



Detector Traces & Event Reconstruction

• Charge rise time is fast (~ 1 us) compared to the phonon rise time (~10-20 us)• Phonon pulse time of arrival allows for event position reconstruction• Event energy reconstructed using optimal filter (time => FFT => fit in freq)• Example of a typical 20 keV event in a Si & Ge ZIP

(very good signal/noise for a 20 keV true recoil energy event)

Si ZIP Ge ZIP



Discrimination of nuclear recoils to electron recoils

Yield = Ionization Energy Total Recoil Energy

Calibration Data

Surface betas are electron recoils with reduced charge collection



Timing provides further discrimination

Use phonon risetimen-recoil is slower than e-recoil

AndCharge to phonon delay

smaller for surface e’sgammas

neutrons

Ejectrons=e- products of incident radiation



Phonon energy in keVPhonon energy in keV

Excellent agreement between data and Monte CarloExcellent agreement between data and Monte Carlo

Source runs also provide energy calibration

Ge ZIP with 133Ba sourceIonization energy in keVIonization energy in keV



Possible Sources of Beta Backgrounds

• Ejected betas from incident gammas. Monte Carlo of detector response and risetime analyses:

50% of our “beta contamination”, but less than 3% of our beta background 1 ejectron per 25k incident gammas appears in the nuclear recoil band

• Radon contamination on the copper cans or the detectors themselves. Alpha analyses (a:b ~ 1:1 for 210Pb) 30-60% of our background.

• K contamination introduced during fabrication & adventitious surface C Ion beam characterization 20-30% of the beta background.



Expected Backgrounds

Source Expected # events (WIMP search Ge data set)

Cosmogenic punch-through neutrons

< 0.3 events (90% CL) given no veto-coincident neutrons

Beta leakageCuts applied to extra Cal. sets

0.67 events (90% CL)

Beta leakageCal. distribution extrapolation

0.88 +/- 0.45



Blind Analysis of WIMP Search Data

Same Tower 1 as run at Stanford: 52.6 live days raw, 19.4 kg-d of Ge after cuts

All detector evaluations and cut definitions were done with calibration runs.Veto anti-coincident WIMP search data in the nuclear recoil region was blinded.



Cuts applied to WIMP search data(all detectors summed)

CUTS Evts

Raw 968,680Data Quality 807,419Q and P thresh 199,338Veto anti-coinc194,088 Single scatters 87,596< 100 keV recoil

13,947Qinner electrode 8,845Pileup cut 8,240Timing cut 1,249N-recoil band 1

Raw Wimp Search Data



After applying Cuts: Wimp search data for individual detectors

Contaminated by calibration source

seen in previous shallow run

Saw no events in nuclear recoil band in Blind Search

But after unblinding,we found a software error: Fit for saturated pulses had been also applied to many unsaturated events.

Correction improves cut efficiency, but Z5 event now passes.

All plots shown are for corrected data.

Si

SiGe

Ge

Ge Ge



Calculate limits

New Limit is 4 x better than Edelweiss

10 x better than CDMS I

Not consistent with WIMPs being the DAMA annual modulation signal



CDMS II Future Plans

Current data run (Mar 25th to Aug 9th, 2004) ~65 live days with 2 towers of detectors (5 Ge ZIPs)Charge thresholds lowered by ~20% (improved electronics grounding) More 133Ba calibration data to improve beta systematic analyses

New analysis methodsImproved Z-position reconstruction5-parameter phonon timing cut w/covariance matrixImprove timing resolution of phonon leading edge by pulse fitting

Warm up Aug-Oct, 2004 to install 5 towers of detectorCryocooler (already tested) to handle the added heat load

Run until Dec 2005 to achieve ~ x 20 increase in sensitivity



Summary

• CDMS has successfully collected data at Soudan Finished the analysis of that first data run

results submitted to PRL: astro-ph/0405033

• No WIMP signal was observed. Current CDMS limits improve by a factor of ~4

(assuming standard DM halo and WIMP scalar interactions)

• No anticipated problems to achieving x 20 improvement

First Results from the Cryogenic Dark Matter Search at the Soudan Underground Laboratory

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