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

Outline What is dark matter and why search for it? Detection Techniques Some Results DAMA-- Yes CDMS-- No Undergraduate Student Participation Dak Matter 10-26-01 R. Dixon

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 Dak Matter 10-26-01 R. Dixon

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. Dak Matter 10-26-01 R. Dixon

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. Dak Matter 10-26-01 R. Dixon

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. Dak Matter 10-26-01 R. Dixon

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. Dak Matter 10-26-01 R. Dixon

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. Dak Matter 10-26-01 R. Dixon

Inventory of the Universe Visible Matter .01 Evidence Telescope observations Composition Ordinary matter-- protons and neutrons Baryonic Dark Matter .05 BBN Matter too dim to see Nonbaryonic Dark Matter .3 Gravity, CMB WIMPs, Axions, Neutrinos Cosmological Dark Matter .6 CMB, Supernova Data Total ~1 Dak Matter 10-26-01 R. Dixon

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 Dak Matter 10-26-01 R. Dixon

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 Dak Matter 10-26-01 R. Dixon

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. Dak Matter 10-26-01 R. Dixon

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 Dak Matter 10-26-01 R. Dixon

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. Dak Matter 10-26-01 R. Dixon

Ge BLIP Ionization & Phonon Detectors Dak Matter 10-26-01 R. Dixon

BLIP TEST DATA Dak Matter 10-26-01 R. Dixon

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) Dak Matter 10-26-01 R. Dixon

Stanford Site, Shield, and Cryostat Dak Matter 10-26-01 R. Dixon

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. Dak Matter 10-26-01 R. Dixon

Icebox and Shielding Dak Matter 10-26-01 R. Dixon

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. Dak Matter 10-26-01 R. Dixon

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. Dak Matter 10-26-01 R. Dixon

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 Dak Matter 10-26-01 R. Dixon

Annual Modulation Dak Matter 10-26-01 R. Dixon

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 Dak Matter 10-26-01 R. Dixon

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 Dak Matter 10-26-01 R. Dixon

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. Dak Matter 10-26-01 R. Dixon

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 Dak Matter 10-26-01 R. Dixon

Detector Fabrication Dak Matter 10-26-01 R. Dixon

BLIP TEST DATA Dak Matter 10-26-01 R. Dixon

Surface Electrons Dak Matter 10-26-01 R. Dixon

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 Dak Matter 10-26-01 R. Dixon

Rise Time Descrimination Dak Matter 10-26-01 R. Dixon

CDMS II Dak Matter 10-26-01 R. Dixon

CDMS Shield Dak Matter 10-26-01 R. Dixon

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Undergraduate Participation Internships for Physics Majors http://ipm.fnal.gov/ Wide Participation But, only about 20 students Dak Matter 10-26-01 R. Dixon

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 Dak Matter 10-26-01 R. Dixon

Conclusions “If you want to find dark matter, why don’t you just go outside at night?” Sam Dixon Mineral Hill, NM Dak Matter 10-26-01 R. Dixon