The Search for Dark Matter The Cryogenic Dark Matter Search (CDMS)

1. The Search for Dark MatterThe Cryogenic Dark Matter Search (CDMS) A Personal Account Roger Dixon

2. Outline • What is dark matter and why search for it? • Detection Techniques • Some Results • DAMA-- Yes • CDMS-- No • Undergraduate Student Participation

3. Case Western Reserve University D.S. Akerib, A. Bolozdynya, D. Driscoll, S. Kamat, T.A. Perera, R.W. Schnee, G.Wang Fermi National Accelerator Laboratory M.B. Crisler, R. Dixon, D. Holmgren Lawrence Berkeley National Lab E.E. Haller, R.J. McDonald, R.R. Ross, A. Smith Nat’l Institute of Standards & Tech. J. Martinis Princeton University T. Shutt Santa Clara University B.A. Young Stanford University D. Abrams, L. Baudis, P.L. Brink, B. Cabrera, C. Chang, R.M. Clarke, P. Colling, A.K. Davies, T. Saab University of California, Berkeley S. Armel, S.R. Golwala, J. Hellmig, V. Mandic, P. Meunier, M. Perillo Isaac, W. Rau, B. Sadoulet, A.L. Spadafora University of California, Santa Barbara D.A. Bauer, R. Bunker, D.O. Caldwell, C. Maloney, H. Nelson, J. Sander, A.H. Sonnenschein, S. Yellin University of Colorado at Denver M. E. Huber Cryogenic Dark Matter Search Collaboration

4. CDMS II

5. Rotation Curve of Solar System

6. Rotation Curve of Our Galaxy

7. Rotations Curves Velocity km/sec Newtonian Prediction Edge of Luminous Disk

8. Big Bang Nucleosynthesis • BBN predicts relative abundance of hydrogen, deuterium, helium, and lithium • Measurement of these abundances

9. Inventory of the Universe • Visible Matter.01 • Evidence • Telescope observations • Composition • Ordinary matter-- protons and neutrons • Baryonic Dark Matter.05 • Evidence • BBN • Composition • Matter too dim to see • Nonbaryonic Dark Matter.3 • Evidence • Gravity, CMB • Composition • WIMPs, Axions, Neutrinos • Cosmological Dark Matter.6 • Evidence • CMB, Supernova Data • Total~1

10. By using information from the Rotation Curves we get Energy Distribution of Dark Matter

11. Candidates • Machos • Particle physics points the way • Supersymmetry (neutralinos) • Axions • Massive neutrinos • Extra Dimensions, curved space, gravitational solutions and on and on . . . Wimpzillas-- people actually get paid to make this stuff up

12. WIMP Direct Search Stategies

13. How Much Dark Matter is in this Room? • Rotations curves ==> .3 GeV/cm3 • Dark Matter in a cubic foot of space in this room assuming each has a mass of 50 GeV-- 170 neutralinos • Total Dark Matter in Solar System = 4.6 X 1017 kg=260 Trillion Buicks • Mass of Sun = 2 X 1030 kg • E = MC2 in Sun ==> 4 years worth of Buicks

14. If WIMPs were produced in the early universe, today they would reside in the halo of the galaxy. An earth-based detector traveling through this halo could detect the particles when they occasionally undergo ‘billiard-ball’ collisions with atomic nuclei. The energy transferred to the scattered nucleus appears as signals in the detector – but how to be certain the signal is due to a WIMP and not some other ordinary ‘background’ particle? In the CDMS experiment, the detectors make all the difference. WIMP detector energy transferred appears in ‘wake’ of recoiling nucleus halo WIMP-Nucleus Scattering bulge sun disk The Milky Way WIMPs in the Galactic Halo

15. Ge BLIP Ionization & Phonon Detectors

16. BLIP TEST DATA

17. Detector performance measured with radioactive sources under laboratory conditions Electron recoils induced from a gamma (photon) source to simulate background events Nuclear recoils induced from a neutron source to simulate WIMP events Clean separation provides rejection of background events due to photons and electrons. Test Particles (Charge Yield)

18. Stanford Site, Shield, and Cryostat

19. 170 gram Ge outer Pb shield scintillator veto Icebox 60 mm Stack of germanium detectors polyethylene outer moderator detectors inner Pb shield dilution refrigerator The CDMS Experiment 60 mm The thermal measurement requires that the detectors be ultra-cold. They are maintained at a temperature of 10 milli-Kelvin by a dilution refrigerator. Because the rate for WIMP scattering is so low, the experiment must also be carefully designed for background suppression: high-purity materials with low radioactivity, shielding against external radiation, an underground site to reduce the flux of cosmic radiation, and a veto to detect residual cosmic rays.

20. Icebox and Shielding

21. The detectors were exposed for a period of several months. The blue dots are the data that remain after rejecting events in coincidence with the cosmic-ray veto or a second detector (see next panel). The circled events are those that fall in the nuclear-recoil band and could be due to WIMPs. However, we also expect nuclear recoils from neutrons that were produced by un-vetoed cosmic rays. These must be estimated and subtracted off to extract the rate due to WIMPs. CDMS Data 1999

22. Neutron Subtraction: Single Scatters vs Multiple Scatters Single-scatter nuclear-recoils are produced by WIMPs or neutrons. Multiple-scatter nuclear-recoils are only produced by neutrons. In addition to the 13 single-scatters, 4 multiple-scatters are observed. The multiple-scatters are used to estimate how many of the single-scatters are due to neutrons. After neutron subtraction, the results are consistent with no single-scatters due to WIMPs.

23. To quantify our non-detection of WIMPs for comparison with other experiments and theoretical predictions, a statistical analysis is performed. For each possible WIMP mass, we determine the largest WIMP size* that could have gone undetected in the data. The regions above the U-shaped curves are ruled out by various techniques. The shaded/dotted regions are predictions from particle physics theories. Limits on WIMP Cross-sections Ge ionization DAMA 1996 CDMS 1999 DAMA 3 Theory DAMA 2

24. Annual Modulation

25. CDMS after background subtraction The DAMA Collaboration runs a competing experiment using a different technique. They look for a seasonal variation in rate expected for WIMPS caused by the Earth’s orbit around the Sun. The amplitude of the modulation correlates with the WIMP-nucleon cross section (effective size). Interesting Times… The best simultaneous fit is shown in red. It corresponds to a WIMP-nucleon cross section too small to explain DAMA’s amplitude but too large to go unseen in CDMS. DAMA 4 year data set

26. The next step for CDMS Larger array & longer exposure Second generation detectors with event positions Deeper site for further reduction in cosmic-ray background DAMA 100kg NaI CDMS (Latest) CDMS Stanford CRESST CDMS Soudan Soudan Mine, Northern Minnesota 2300’ depth Genius Ge 100kg 12 m tank MINOS CDMS II Sensitivity goals of future experiments Soudan II Looking Ahead

27. W/Al QET Sensors Signals from three of four phonon sensors (largest signal arrives first,etc) Ionization signal defines start time

28. Transition Edge Sensors • Steep Resistive Superconducting Transition • Voltage bias is intrinsically stable • W Tc ~ 70-90 mK • 10-90% <1 mK R unitless measure of transition width T

29. Detector Fabrication

30. BLIP TEST DATA

31. Surface Electrons

32. Rise Time Cuts beta/neutron discrimination better than 20:1 Gammas (high Q/P), neutrons shifted slitghtly higher Electrons (low Q/P) (b) Mu-coincident with RT cut Mu-anitcoin with RT cut (a) Mu-coincident with RT cut Mu-anitcoin without RT cut

33. Rise Time Descrimination

34. CDMS II

35. CDMS Shield

43. Undergraduate Participation • Internships for Physics Majors • http://ipm.fnal.gov/ • Wide Participation • But, only about 20 students

44. Students and Activities on CDMS • Jamie Lush, University of South Dakota (1997) • Worked on software for testing electronics and power supplies • Steven Furlanetto, Carlton College • Simulation software • Theodossis Trypiniotis, Cambridge (1999) • Simulations • Shahin Rahman, Washington University (2000) • CDMS/DAMA Cross-section calculations • CDMS/DAMA Problem (2000) • Daniel Osborn, Harvey Mudd • Priscilla Payan, UCLA • Ingyin Zaw, Havard

45. Conclusions • “If you want to find dark matter, why don’t you just go outside at night?” Sam Dixon Mineral Hill, NM