Status of the Cryogenic Dark Matter Search (CDMS) Experiment Bruno Serfass University of California, Berkeley for the CDMS Collaboration Rencontres de Moriond – March 2005

2 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 Florida L. 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…

3  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

4 Ionization Yield  E Q /E R Y~ 1 for electron recoils Nuclear Recoils ( 252 Cf)  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 ( 252 Cf) 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 ( 133 Ba) Bulk Electron Recoils ( 133 Ba)

5 1  tungsten 380  x 60  aluminum fins The ZIP Ionization & Phonon Detectors Q ou ter Q inner z y 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

6 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…

7 Detector response  4 phonon pulses  2 charge pulses (Q inner, Q outer ) 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 Cd Al foil:  22 kev Delay Plot We can correct the phonon energy/timing position dependence

8 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 event Surface 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

9 CDMS II at Stanford and at Soudan Log 10 (Muon Flux) (m -2 s -1 ) Stanford Underground Facility (SUF) Depth (meters water equivalent) 500 Hz muons in 4 m 2 shield  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: (2003) Soudan Mine 1 per minute in 4 m 2 shield  in Soudan Mine (Minnesota)  Depth 713 m (2090 mwe)  Reduce neutron background: ~1/kg/day to ~ 1/kg/year

10 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 LeadPolyethylene mu-metal (with copper inside) Ancient lead

11 Summary of data taking at Soudan  Oct 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 ( 133 Ba, 252 Cf)  Mar – 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

12 Run 118 (Tower 1): Energy calibration with 133 Ba source  Use 356 keV 133 Ba 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 keV Phonon energy in keV MC data

13 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 keV Z2, Z3, Z5 thresholds at 10 keV Run 118

14 WIMPs search data with Ge detectors (Run118)  Yellow points from neutron calibration Charge Yield Prior to timing cutsAfter timing cuts  Blue points from WIMP search data (Z2, Z3, Z5) Recoil energy (keV) Charge Yield Expected background:  0.7 ± 0.35 mis-identified surface electron recoils  ~0.07 unvetoed neutrons Event

15 CDMS limit from Soudan  Exposure after cuts of 52.6 kg-d raw exposure with Ge ≈ 20 kg-days for recoil energies between 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/c 2  Hard to accommodate DAMA annual modulation effect as a WIMP signal! DAMA CDMS SUF EDELWEISS CDMS Soudan Minimum of the limit curve: 4 x cm 2 at 90% C.L for a WIMP mass of 60 GeV/c 2

16 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, (2004)  More details PRD submission in March Current CDMS limit

17 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

19 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, (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  ~ cm 2  Submitted Development Project proposals

20 Backup slides

21 Electrothermal Feedback  Voltage bias supplied Joule heating P = V 2 /R  Quasi particles heat up W  T   R   P   P   T  Stability ElectroThermal Feedback R T  Measure reduction in Joule heating by change in current  I = V/R  E =  I  V dt

Nuclear recoils in Ge ZIP Counts/ (keV kg day) Recoil Energy (keV) Expectations from simulation Data Nuclear recoils in Si ZIP Expectations from simulation Recoil Energy (keV) Counts/ (keV kg day) Data Phonon calibration does not depend on whether the event is a nuclear recoil or electron recoil

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

24 Theory survey - earlier MSSM Baltz & Gondolo PRD (2003) Kim,Nihei,Roszkowski, hep-ph/ Baltz & Gondolo hep-ph/

25 Constrained MSSM and relax GUTs Baer et al, hep-ph/ Chattopadhyay et. al, hep-ph/ Ellis et al, hep-ph/ Bottino, et al hep-ph/

26 mSUGRA and Split Supersymmetry Baltz & Gondolo hep-ph/ A. Pierce, hep-ph/ & G. F. Giudice and A. Romanino hep-ph/

27 Neutron Proton Spin dependent WIMP-nucleon Interactions Preliminary