1. New Results and Status of the Cryogenic Dark Matter Search
Inner Space Outer Space,
Fermilab, June 1999 1

2. CDMS Collaboration Case Western Reserve University
D. S. Akerib, A. Bolozdynya,
T.A. Perera, R.W. Schnee
Fermi National Accelerator Laboratory
M.B. Crisler, R. Dixon, S. Eichblatt
Lawrence Berkeley National Laboratory
E.E. Haller, R.R. Ross, A. Smith
Lawrence Livermore National Laboratory
P.D. Barnes, Jr.
National Institute of Standards and Technology
K.D. Irwin, J. Martinis
Princeton University
T. Shutt
Santa Clara University
B.A. Young
San Francisco State University
J. P. Castle, F.P. Lipschultz, B. Neuhauser, P. Shestople
Stanford University
P.L. Brink, B. Cabrera, R.M. Clarke,
P. Colling, A.K. Davies, S.W. Nam, T. Saab
University of California, Berkeley
A. Da Silva, R.J. Gaitskell, S.R. Golwala, J. Hellmig, J. Jochum, B. Sadoulet, A.L. Spadafora,
University of California, Santa Barbara
D.A. Bauer, D. O. Caldwell, H. Nelson, A.H. Sonnenschein, S. Yellin
University of Colorado at Denver
M. E. Huber 2

3. Direct Search for non-Baryonic Dark Matter in form of WIMPs
Short review of detectors and site
Results from the present run
No longer limited by electron background
Beginning to probe region allowed by DAMA
Limited only by neutron production due to shallowness of site
Future plans and conclusions 3

4. Nuclear Recoil Discrimination Ionization yield per unit recoil energy depends strongly on type of recoil
WIMPs (and neutrons) produce nuclear recoils
Most background sources (photons, electrons, alphas) produce electron recoils
Unfortunately, detectors have thin (~20 mm) "dead layer" of reduced charge-collection -- surface events (primarily electrons) to mimic nuclear recoils
Electron recoils
Nuclear recoils 4

5. 2 Detector Technologies
Low field (~volts/cm) ionization measurement with segmented contacts
Calorimetric measurement of total energy: ~1 µK/10 keV (2 µV/10 keV) at 20 mK
165 g Ge target, 1.2 cm x 6 cm ø
Collect athermal phonons :
XY position imaging
Surface (Z) event veto on pulse shape
100 g Si target, 1 cm x 7.5 cm diameter
250 g Ge target tested but not yet run in low-background environment
Photolithographic fabrication
 Assembly line production
Z-sensitive Ionization and Phonon-mediated
Berkeley Large Ionization and Phonon-mediated detector 5

6. Site, Shield, and Cryostat
Stanford Underground Facility
17 mwe of rock
hadronic component down by >1000
muon flux down by ~5
Muon Veto and Shield
muon veto >99.99% efficient
25 cm polyethylene reduces muon-induced neutron flux from rock and lead by factor >100
15 cm Pb reduces photon flux by factor >1000
Icebox
radiopure cold volume (10 kg)
additional internal (ancient) lead shielding
may add internal polyethylene
can hold up to 18 detectors A c t i v e M u o n V
Detectors
Inner Pb shield
Polyethylene
Pb Shield 6

7. Current Data Run (Fall 1998 - Summer 1999)
6 x 165g Ge = 1 kg Ge
BLIPs 3-6
Improved ionization contacts to reduce effect of dead layer
Refined cleanliness regimen
Close-packed (3.5 mm spacing) for self shielding
2 mm passive Ge shielding
5 months (11/98-4/99), 35 raw livedays
~1 kg-day exposure per detector after cuts (removes periods of noise, pile-up of events, unvetoable events, events outside inner ionization electrode fiducial volume)
BLIPs 1-2
Worse ionization contacts, known electron contamination
Run primarily to confirm that contamination was from housing
Ge shielding
BLIP 4
BLIP 3
BLIP 5
BLIP 6
exchange housings
BLIP 2
BLIP 1 7

8. BLIPs 3, 4, 5, and 6 Tower Wiring heat sinking
holds cold FETs for amplifiers
Inner Ionization
Electrode
Outer Ionization
Electrode
Passive Ge shielding 8

9. Results from the Current Run
+ single-scatter x multiple-scatter
Obvious contamination in B3: m-anticoincident singles rate >2x any other detector (prototype) -> not used for analysis
Only 6 single-scatter veto-anticoincident nuclear recoil candidates in B4-B6
1 multiple-scatter n-n in B5/B6, good event, right on nuclear recoil lines -> most events are neutrons
If cuts relaxed slightly, get 2nd good n-n scatter 9

10. Additional Evidence that Events are Neutrons
Small Statistics: time will tell
Spectrum agrees well with Monte Carlo simulation for n's
Fair consistency with Si ZIP data from previous run
Expect equal number of neutrons in 1.5 kg-d in Si as in 3.1 kg-d Ge
Detect 2-4 in Si and 7 in Ge
Kolmogorov-Smirnov test indicates 72% chance of agreement
mostly neutrons
mostly
electrons 10

11. Neutrons are likely created outside Muon Veto
Efficiency of veto appears too high to allow leakage of internally produced neutrons
Mapping for holes found worst areas have 1/3 light collection of the best areas
Rate consistent with expectation from external neutrons
Predicted rate (unsure to factor of 3) ~2x high (10.7 events predicted)
Some fraction vetoed? (MC work to come)
Polyethylene
Pb Shield
Muons passing through detector: 99.99% vetoed
Detect ~600 neutrons from inside copper- need 99.9% vetoed
Detect ~20 n's produced in lead- need 95% vetoed
Active Muon Veto 11

12. Current CDMS Limits Starting to probe DAMA 90% allowed region
PRELIMINARY
Neutron Subtraction
Use conservative limit on measured number of n's in Si (2), multiple n's in Ge (1)
Use Monte Carlo simulation to fix ratio of Si n's, Ge n-n's, to single Ge n's
3 events expected, 6 seen above 10 keV
PRELIMINARY
Changing cuts to improve high-energy efficiency
Using conservative 10 keV recoil threshold
with subtraction
without subtraction 12

13. Comparison to WIMP models
M = 40 GeV, s=9.6x10-42 cm2 (upper left point of DAMA 90% allowed region) is excluded by CDMS at 90%
M = 59 GeV, s=7x10-42 cm2 (center of DAMA 90% allowed region) is allowed by CDMS at 90%
M = 40 GeV, s=9.6x10-42 cm2
M = 59 GeV, s=7x10-42 cm2
CDMS nuclear recoil candidates 13

14. Can We Reach the CDMS Sensitivity Goals?
Solutions for neutrons
Add internal/external moderator
Improve muon veto
Increase exposure and subtract background
Deep Site -- Soudan
Current Run
Some improvement from 4x exposure, ext. moderator
Next Run (Start Fall 1999)
30 kg days Ge / 12 kg days Si -> expect ~60 events, or 0.015/keV/kg/day at 90% CL after background subtraction
Hope to reduce rate
100 kg-d 0.01/keV/kg/day
Ge diodes
500 kg-d typ
2500 kg-d /keV/kg/day 14

15. Future Plans Stanford (shallow site), Summer 1999
Continue running close-packed BLIP detectors to deconvolve effects of improved electrodes, close-packing, cleanliness and passive shielding
Run at low charge bias to separate electron background from photons
Gamma calibration to help measure effect of improved electrodes
Add veto diagnostic to help confirm origin of neutrons
Stanford (shallow site), Fall Spring 2000
Likely add internal and/or external neutron moderator
Close-packed Ge ZIP package (3x250g Ge & 3x100g Si) should probe MSSM
Deep Site: 2000 and on...
Deep site to reduce cosmogenic neutron flux: Soudan mine
42 detectors: >10 kg of mass
Experimental housing and new Icebox under construction
0.0003/kg/keV/day (1 event/detector/year) via reduced background fluxes, demonstrated detector improvements, background subtraction 15

16. Conclusions
CDMS has sight of its projected Stanford goal: 0.01/keV/kg/day
Fourfold strategy of detector improvements, close-packing, cleanliness, and shielding successful in addressing electron backgrounds
Photon background below goal; no statistical subtraction needed
Neutron background higher than originally expected. However, even without rate reductions, goal can nearly be reached.
Infrastructure for extended running of cryogenic detectors in place
Current results beginning to probe region allowed by DAMA
Next run should probe MSSM! 16

17. Spectra from the Current Run
PRELIMINARY
Analysis threshold conservatively placed at 10 keV recoil energy
Nuclear recoil acceptance drops at energies above 20 keV due to overly conservative inner electrode fiducial volume cut
Ga x-ray
Nuclear Recoil Efficiency 17

18. Neutron Spectra Predictions of Monte Carlo simulations agree very well
We understand our efficiencies for detecting nuclear recoil events
We understand the interactions of muons in the shield 18