1. The Search for Dark Matter The 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
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3. Cryogenic Dark Matter Search Collaboration
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
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4. CDMS II
The Cryogenic Dark Matter Search (CDMS) uses detectors that measure both phonon energy and ionization. Several detector designs have been investigated by the CDMS collaboration and one of these will be described here.
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5. Rotation Curve of Solar System
The evidence for dark matter comes primarily from it’s apparent gravitational interaction with the visible universe. To see this we look first at the solar system to remind ourselves of the simplicity and effectiveness of Newton’s laws in explaining the motions we observe there. The planets move around the sun in perfect congruence to the mathematics which describe Newton’s general principles of motion as shown in figure 1. There is no evidence that anything might be amiss. The velocity of the planets falls as
just as predicted by Newton.
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6. Rotation Curve of Our Galaxy
However, if we make the same kind of observations for the motion within our own galaxy we run into immediate problems. The rotation curve of the galaxy is shown in figure 2. The velocity function appears to be better described by a constant rather than the inverse square root distance behavior expected. This leads us to ask what we might expect of the solar system if it were filled with a large amount of dust out to great distances. Note that the curve for this possibility is shown in figure 1. In that case we get a constant distribution of velocities in space with distance from the center much as is observed for the galaxy. Could our galaxy be filled with some invisible material which behaves gravitationally as a dust cloud? That is the evidence.
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7. Rotations Curves Velocity km/sec Newtonian Prediction
Edge of Luminous Disk
This all becomes even more clear if we look at distant galaxies and measure their rotation curves. For this case we can measure the velocity distribution of the gas to distances much greater than the extent of the luminous disk. Surprisingly we see no evidence that the velocity ever begins to decrease with distance. The implication is that the invisible halo is much larger than the visible part of the galaxy, and the amount of matter making up the halo is more than 10 times the visible matter.
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8. Big Bang Nucleosynthesis
BBN predicts relative abundance of hydrogen, deuterium, helium, and lithium
Measurement of these abundances
Let us consider the Big Bang theory to begin our theoretical considerations. One of the major triumphs of the theory is that it predicts the relative abundances of the lightest of elements based on the rates of the nuclear reactions which produce the helium, deuterium and lithium from the primordial hydrogen. Observations confirm these ratios very accurately. This allows us to use the theory and measurements of abundances to place constraints on the total amount of baryonic matter in the matter in the universe. When we do this we find that
While the fraction of visible matter is only.
Hence, we can conclude that much of the baryons in the universe may, in fact, be invisible to us.
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9. Inventory of the Universe
Visible Matter
Evidence
Telescope observations
Composition
Ordinary matter-- protons and neutrons
Baryonic Dark Matter
BBN
Matter too dim to see
Nonbaryonic Dark Matter
Gravity, CMB
WIMPs, Axions, Neutrinos
Cosmological Dark Matter
CMB, Supernova Data
Total ~1
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10. Energy Distribution of Dark Matter
By using information from the Rotation Curves we get
It is an easy matter to compute the energy density of the halo from basic principles. We find that
By using the best determinations we have for the luminous mass and the rotation curves, we determine that
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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
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12. WIMP Direct Search Stategies
Detection of WIMPs can be accomplished in two manners. The first is called indirect detection and relies on detecting the products of the WIMP annihilation process. This method may be enhanced if the WIMPs tend to cluster in the center of the sun or the earth. One would look for the neutrinos from the annihilations of the particles and antiparticles. This method will not be considered further here.
Instead, we will discuss direct detection of WIMPs. Since the particles are weakly interacting they are expected to scatter off of a nucleus with a weak scattering cross-section. It is these nuclear recoil events we hope to detect in our direct detection experiments.
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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 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
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14. WIMPs in the Galactic Halo
detector
energy transferred appears in
‘wake’ of recoiling nucleus
WIMP-Nucleus Scattering
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.
halo
bulge
disk
sun
The Milky Way
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.
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15. Ge BLIP Ionization & Phonon Detectors
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16. BLIP TEST DATA
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17. Test Particles
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.
(Charge Yield)
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18. Stanford Site, Shield, and Cryostat
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19. Stack of germanium detectors
The CDMS Experiment
polyethylene
outer moderator
detectors
inner Pb
shield
dilution
refrigerator
Icebox
outer Pb shield
scintillator
veto
170 gram Ge
60 mm
Stack of germanium detectors
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.
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20. Icebox and Shielding
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21. CDMS Data 1999
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.
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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.
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23. Limits on WIMP Cross-sections
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.
Ge ionization
DAMA 1996
CDMS 1999
DAMA 3
Theory
DAMA 2
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24. Annual Modulation
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25. Interesting Times… 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).
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
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26. Sensitivity goals of future experiments
Looking Ahead
DAMA 100kg NaI
CDMS Soudan
CDMS Stanford
Genius Ge 100kg 12 m tank
CDMS (Latest)
CRESST
Sensitivity goals of future experiments
The next step for CDMS
Larger array & longer exposure
Second generation detectors with event positions
Deeper site for further reduction in cosmic-ray background
Soudan Mine, Northern Minnesota
2300’ depth
CDMS II
Soudan II
MINOS
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27. W/Al QET Sensors Signals from three of four Ionization phonon sensors
(largest signal
arrives first,etc)
Ionization
signal defines
start time
Figure 10 shows the CDMS ZIP detectors which emply the tungsten transition edge sensors to measure the phonon energy in addition to the ionization energy which is measured by putting a small bias voltage across the crystal. The detectors are made to operate at 20 mk for the crystal, but the phonon sensors on the surface are maintained at 60 to 90 mk.
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28. Transition Edge Sensors
Steep Resistive Superconducting Transition
Voltage bias is intrinsically stable
• W Tc ~ mK
• 10-90% <1 mK R
unitless measure
of transition width T
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29. Detector Fabrication
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30. BLIP TEST DATA
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31. Surface Electrons
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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
(a)
Mu-coincident
with RT cut
Mu-anitcoin
without RT cut
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33. Rise Time Descrimination
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34. CDMS II
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35. CDMS Shield
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36. Dak Matter
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37. Dak Matter
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38. Dak Matter
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39. Dak Matter
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40. Dak Matter
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41. Dak Matter
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42. Dak Matter
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43. Undergraduate Participation
Internships for Physics Majors
Wide Participation
But, only about 20 students
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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
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45. Conclusions
“If you want to find dark matter, why don’t you just go outside at night?”
Sam Dixon
Mineral Hill, NM
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