# Presentation: 526681

## CDMS II at Soudan - The Hunt for WIMPs

-                                       CDMS Collaboration

-                                               CDMS ZIP Detectors
- CDMS ZIP detector is 250g (100g) germanium (silicon) ‘puck’, which has two charge collection channels and  four ballistic phonon collection channels. When particles recoil off an electron or nucleus in the target material (Ge or Si), electron-hole pairs and phonons are generated. Electron-hole pairs are separated under external electric field, and are collected with charge integrators in inner and/or outer electrodes.
- Phonon sensors are Al ballistic phonon trapping fins and W transition-edge sensors  (TES). Quasiparticles created by phonons captured in Al fins diffuse into W TES. The resistance change in TES gives a current change (signal) in electrothermal feedback circuit (shown above). And the signal is read out with SQUID system.

-                  WIMPs as Non-Baryonic Cold Dark Matter
- There is evidence in both astronomical
- observations and modern cosmology
- that most matter in the universe is
**dark and non-baryonic**.
- Weakly Interactive Massive Particles
- (WIMPs) as cold dark matter candidates
- arise naturally from supersymmetry.
- WIMPs refer to a more general class of
- particles, including neutralinos, that are
- relics left over from the Big Bang.
- Annihilation stops (****~ m**** when
- number density drops to the point that
- **********A**~ n********A v
- i.e., annihilation too slow to keep up with
- Hubble expansion (“freeze out”)
- Leaves a relic abundance:
********h2 10-27 m3 s-1 ****A vfr
- Cryogenic Dark Matter Search (CDMS) experiment looks for WIMPs,  which deposit few to few tens of keV energy when they elastically recoil off Ge or Si nuclei .Expected rate is < 1 event/kg/d. Underground experimental environment, active scintillation muon veto, shields of lead, polyethylene and copper provide the required low background conditions. And more important, CDMS detectors themselves have event by event background rejection capability.

- _Brown University_
- M. Attisha, R.J. Gaitskell, J.-P. Thompson
- _Case Western Reserve University                      _D.S. Akerib, M.R. Dragowsky, D. Driscoll,          S. Kamat, T.A. Perera, R.W. Schnee, G.Wang
- _Fermi National Accelerator Laboratory_
- D. A. Bauer, M.B. Crisler, R. Dixon,                    D. Holmgren, E. Ramberg
- _Lawrence Berkeley National Laboratory_
- J.H. Emes, R.R. Ross, A. Smith
- National Institute of_ _
- _Standards and Technology_
- J. Martinis
- _Princeton University_
- T. Shutt_ _

- Cryogenic
- Dark
- Matter
- Search

- CDMS Background Discrimination

- Rejection of surface electrons based on ionization yield alone is >90% above 10 keV
- Rejection of electrons based on risetime of phonon pulses is >90% while keeping >55% of the neutrons
- Actual overall rejection of electrons is >99%, twice as good as in CDMS II proposal

- Stanford Underground Facility (SUF) at 17 mwe of rock
- Active scintillator + gamma and neutron shielding + radio-pure inner volume
- Event-by-event nuclear recoil discrimination by using 6 ZIP detectors
- 4 Ge (250g each) and 2 Si (100g each) detectors
- 3V data set, 93 real days, 67 live days, and a total of 4.7 million events
- 6V data set, 74 real days, 52 live days, under analysis

- CDMS II SUF RUN

- Neutron Multiple Scatters

- 2 triple-scatter (filled circles) and 1 non-nearest-neighbor double-scatter () NR candidates 5-100 keV
- Ignore nearest-neighbor doubles because possible contamination by surface electrons
- Expect ~16 single-scatter neutrons per 3 multiple scatters
- Implies many (or all) of 20 single-scatter WIMP candidates are neutrons

- _Santa Clara University_
- B.A. Young
- _Stanford University_
- L. Baudis, P.L. Brink, B. Cabrera,           C. Chang, T. Saab, W. Ogburn
- _University of California, Berkeley_
- M.S. Armel, A. Lu, V. Mandic, P. Meunier, N. Mirabolfathi, W. Rau, B. Sadoule**t**
- _University of California, Santa Barbara_
- R. Bunker,** **D.O. Caldwell, R. Ferril,        R. Mahapatra, H. Nelson, J. Sander,      C. Savage, S. Yellin
- _University of Colorado at Denver_
- M. E. Huber_ _
- _University of Minnesota_
- P. Cushman, L. Duong, A. Reisetter

- Ionization Yield (ionization energy per unit recoil energy) depends strongly on type of recoil
- Most background sources (photons, electrons, alphas) produce electron recoils
- WIMPs (and neutrons) produce nuclear recoils
- Phonons from interaction are optical phonons, these phonons decay into high energy acoustic phonons
- Phonon propagation velocity is a strong function of phonon frequency.
- Surface impurities in Si or Ge have a soft electronic structure with low excited energy levels. Scattering rate of phonons with these impurities is several orders higher compared to the bulk closed-shell impurities
- Surface events produce lower-frequency phonons in much shorter time
- Faster phonons result in a shorter risetime of the phonon pulse
- Risetime helps eliminate the otherwise troublesome background surface events
- Nuclear recoil is identified with both yield and risetime

- Underground experimental environment reduces muon flux by orders of magnitude
- Active scintillation paddles veto remaining muon-induced events
- Lead, polyethylene and pure copper shields suppress radioactive background
- Extensive precautions against contamination  during detector storage and handling
- Neutron background is estimated by looking at multiple scattering events and relative event rates in Ge and Si (similar neutron cross sections but coherent WIMP cross section x5 higher in Ge than in Si).
- Detectors provide near-perfect event-by-event discrimination against otherwise dominant electron-recoil backgrounds (99.95% against photons, 99% against electrons).

- CDMS II at Soudan

- Flat galactic rotation curves     ****m ≥ 0.1
- Cosmic Microwave Background ********+********m ≈ 1
- WMAP (astro-ph/0302209)          ****m ≈ 0.29 ± 0.07
- Big Bang Nucleosynthesis (astro-ph/0001318)                  ****b= 0.040 ± 0.005
- Cosmic Microwave Background (astro-ph/0302209)                                   ****b= 0.047±0.006

- Four phonon channels A, B, C, D
- Phonons travel across the detector in quasi-ballistic mode, average phonon speed in Si (Ge) crystal of 0.25 (0.12) cm/s results in measurable delays between the pulses of the 4 phonon channels
- x, y coordinates of interaction location can be reconstructed with four phonon channel’s pulse start time
- Z-coordinate reconstruction is in progress
- Frequency down conversion of phonons depends on depth of interaction and this is the risetime handle of the phonon pulses

- SUF Run Muon-Anticoincident Data

- Now operating at deep site: Soudan mine, Minnesota, 713 m (2090 mwe)
- Muon flux reduced by > factor 30,000
- Neutron background reduced from ~1 / kg / day to ~1 / kg / year
- First data run with Tower 1 (10/11/2003-1/11/2004): 56 live-days, ~30 kg-days
- First x10 improvement in sensitivity ~ 0.1 evt/kg/day already achieved!
- Second data run with two towers beginning: expect 90 live-days, 50 kg-days.
- Full complement of detectors (5 towers) installed later this year
- During 2005 data run, expect to accumulate 600 kg-days
- Final expected sensitivity ~ 0.01 evt/kg/day
- Proposal to funding agencies for two additional detector towers, 3 more years of running => sensitivity ~0.002 evt/kg/keV/day.

**CDMS II at Soudan - The Hunt for WIMPs**
**Dan Bauer, Mike Crisler, Roger Dixon, Don Holmgren, Erik Ramberg**
**Fermilab**

- WMAP

- Upper Limits on WIMPs

- Beginning to challenge supersymmetry models
- Exclude DAMA most likely point (x) at 99.8% CL

- 90% CL upper limits assuming standard halo, *A*2 scaling

- 10-6 pb

- Active Background rejection is key to future progress
- CDMS II in best position to establish WIMP discovery

- MSSM