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

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

A Personal Account

Roger Dixon

Dak Matter 10-26-01R. Dixon

Outline

• What is dark matter and why search for it?• Detection Techniques• Some Results

– DAMA-- Yes

– CDMS-- No

• Undergraduate Student Participation


Cryogenic Dark Matter Search Collaboration

Case Western Reserve UniversityD.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 LabE.E. Haller, R.J. McDonald,

R.R. Ross, A. Smith

Nat’l Institute of Standards & Tech.J. Martinis

Princeton UniversityT. Shutt

Santa Clara UniversityB.A. Young

Stanford UniversityD. Abrams, L. Baudis, P.L. Brink, B. Cabrera, C. Chang, R.M. Clarke,

P. Colling, A.K. Davies, T. Saab

University of California, BerkeleyS. Armel, S.R. Golwala, J. Hellmig, V. Mandic, P. Meunier, M. Perillo Isaac, W. Rau, B. Sadoulet, A.L. Spadafora

University of California, Santa BarbaraD.A. Bauer, R. Bunker,

D.O. Caldwell, C. Maloney,

H. Nelson, J. Sander,

A.H. Sonnenschein, S. Yellin

University of Colorado at DenverM. E. Huber


CDMS II


Rotation Curve of Solar System

10000

20000

30000

40000

50000

Distance from the Sun (Meters x 1012)

1 2 3 4 5 6

PlutoUranus

Saturn

Jupiter

Mars

Earth

Venus

Mercury

Neptune

Prediction for dust filled solar system (no sun)

Newtonian prediction for sun alone (no dust)


Rotation Curve of Our Galaxy

Satellite Galaxies

GC

MC

GC

CO

GC

300

40

200

100

806020

Newtonian Prdiction


Rotations Curves

Edge of Luminous Disk

Newtonian Prediction

Vel

ocit

y km

/sec


Big Bang Nucleosynthesis

• BBN predicts relative abundance of hydrogen, deuterium, helium, and lithium

• Measurement of these abundances

Ωbaryon ≤ .05


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


Energy Distribution of Dark Matter

GM =v2r

v tot(r) = vd2 (r) +vh

2 (r)[ ]12

ρ r( ) =1

4πGr 2

d

drrvh

2 r( )( )

ρ0 = .3 → .6 GeV • cm−3

v tot(r) ≈200 km⋅sec−1

v21

2 ≈270 km• sec−1

By using information from the Rotation Curves we get


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


WIMP Direct Search Stategies


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


WIMPs in the Galactic Halo

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.

halo

bulge

disksun

The Milky Way

WIMP detector

energy transferred appears in ‘wake’ of recoiling nucleus

WIMP-Nucleus Scattering


Ge BLIP Ionization & Phonon Detectors


BLIP TEST DATA


Test Particles

Detector performance measured with radioactive sources under laboratory conditions

Electron recoils induced from a gamma (photon) source to simulate background eventsNuclear recoils induced from a neutron source to simulate WIMP events

Clean separation provides rejection of background events due to photons and electrons.

(Cha

rge

Yiel

d)


Stanford Site, Shield, and Cryostat


The CDMS Experiment

polyethyleneouter moderator

detectors inner Pbshield

dilutionrefrigerator

Iceboxouter Pb shieldscintillator

veto

60 m

m

170 gram Ge

60 mm

Stack of germanium detectorsThe 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.


Icebox and Shielding


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.


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.


Limits on WIMP Cross-sectionsTo 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.

CDMS 1999

DAMA 3DAMA 2

DAMA 1996

Ge ioniza

tion

Theory


Annual Modulation


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


Looking Ahead

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 IISoudan II

MINOS

DAMA 100kg NaI

CDMS Soudan

CDMS Stanford

Genius Ge 100kg 12 m tank

CDMS (Latest)

CRESST

Sensitivity goals of future experiments


W/Al QET Sensors

CAB

D

n,γ Eph + Ee-h(WIMPS)

20mKbase

100 g Si and 250 g Ge Crystals

1 cm Thick X 3” Dia

Qinner

Qouter

Vqbias

-30 -20 -10 0 10 20 30 40 50 60 70-100

0

100

200

300

400

500

600

700Ionization and Phonon Event in 100 g Si Detector

Time [µsec]

Ionizationsignal defines

start time

Signals fromthree of fourphonon sensors(largest signalarrives first,etc)

τ ph−Ge ~4τ ph− Si ~220 μs


Transition Edge Sensors• Steep Resistive Superconducting Transition

• Voltage bias is intrinsically stable

R

T

• W Tc ~ 70-90 mK• 10-90% <1 mK

α =dR

dT

R

Tunitless measure

of transition width

Rshunt

Ibias

W ETF-TES

SQUIDArray

The Joule heating produced by bias

PJ =VB

2

R⇒ PJ ↓ whenR↑

is stable whereas for current bias

PJ =I B2 R ⇒ PJ ↑ whenR↑

which is intrinsically unstable


Detector FabricationAl/W Grid

60% Area Coverage

37 - 5 mmSquares 888 X 1 µm

tungsten TESin parallel

Aluminum Collector Fins

8 Traps


BLIP TEST DATA


Surface Electrons


Rise Time Cuts

Gammas (high Q/P),neutrons shifted slitghtly

higher

Electrons (low Q/P)

beta/neutron discrimination

better than 20:1

(a) Mu-coincident

with RT cutMu-anitcoin

without RT cut

(b) Mu-coincident

with RT cutMu-anitcoinwith RT cut


Rise Time Descrimination

Beta source events

gamma calibration

nuclearrecoils

slowrisetime

fastrisetime

nuclearrecoils

gammas

gammas

betas betas


CDMS II


CDMS Shield


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are needed to see this picture.




















Undergraduate Participation

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


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


Conclusions

• “If you want to find dark matter, why don’t you just go outside at night?”

Sam Dixon

Mineral Hill, NM

Documents

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