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

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

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

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

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

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

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

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

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

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

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

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

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

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 0.0003/keV/kg/day 14

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 1999 - 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

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

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

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