CDMS

(Cryogenic Dark Matter Search)

Long Duong (University of Minnesota)Trinity School Seminar

Jan 14, 2004

• Introductory remarks• Outline of physics concepts• Description of the experiment• Results

~ 40 physicists/students from 8 institutions work on CDMS (experimental site is the Soudan mine in northern MN)

UCB (UC at Berkeley), UCSB (UC at Santa Barbara), Santa Clara,Stanford, Colorado U, NIST (Colorado), LBL (Lawrence Berkeley Lab) CWRU (Case Western Reserve), Brown, UMN (U of Minn in Mpls), FNL (Fermi National Lab)

Physics Concepts

• Atomic theory of matter

• Standard model of physics

• Gravity

• Light matter (Dark matter)

• Temperature (evaporative cooling)

• Ionization (or release of bound electrons)

• Phonon (or lattice vibration of a solid)

Electric charge ofvarious particles e = -1p = +1n = 0u = +2/3d = -1/3

Standard Model of Physics

* everything is made of quarks and leptons * interactions occur via exchange of mediators

Quarks(charge) Leptons (charge)u c t (+2/3) e d s b (-1/3) e

Strength Mediator Participants 1 photon all nonzero 10^(-11) W+, W-, Z0 all

Higgs boson all 1000 gluon quarks only 10^(-40) graviton all

Gravity: least understood of all the forces

F (on sun) = G * M(bulge) * M(sun) / (R * R) + F (from other stars) = M(sun) * (V * V) / R

=> V = sqrt[ (G/R) M(bulge) ] + …

Rotation curve of the Milky Way spiral galaxy cannot be explained only by the observed light matter. Somepossible explanations:

* gravity is modified on large scale* existence of a “dark” matter halo(dark matter hypothesis favored from other evidence,e.g. weak gravitational lensing studies of sky)

Layout of CDMS experiment

Dilution Refrigerator Icebox

Ebox

Thermal stages of a dilution refrigerator:room temp. 293 K (red) OVC (outer vacuum chamber)liquid N 77 K (yellow)liquid He 4 K (green) IVC (inner vacuum chamber)pot 1 K (light blue)still 0.6 K (light blue)cold Plate 0.05 K (dark blue)mixing chamber 0.01 K (black)

Detectors (ZIP)(at 0.01 K stage)

Temperature (kinetic theory of thermodynamics)

thermodynamic equilibriumliquid temperature = Tgas pressure = P

pump on gas phaseliquid temp < Tgas pressure < P

Temperaturea measure of the average energy inthe random motionof a very large (~10^25)number of atoms thatmake the gas, liquid orsolid phase of a material

Germanium/Silicon crystals as particle detectors

dark matter particle in the galactic halo WIMP (Weakly Interacting Massive Particle)

Two quantities can be measured when a WIMP (or any particle) scatters offsome constituent of a crystal:

1) Lattice vibrations (phonons)2) Electronic shells are excited

and electrons are freed from being bound to a particularnucleus (ionization)

Q inner

Q outer

A

B

D

C

Rbias

I bias

SQUID array Phonon D

Rfeedback

Vqbias

Each ZIP detector has 4 phonon (A, B, C, D)and 2 ionization sensors (Qi, Qo)

Picture of a ZIP detector

phononsensors

Copper caseis thermallyconnectedto the 0.01 Klayer of theicebox

Two kinds of interactions with a ZIP constituent

* scattering from an electron (electron recoil)* scattering from a nuclei (nuclear recoil)

electron recoils(data from Cs source)

nuclear recoils(data from Cf source)This is the region CDMS uses to searchfor WIMPs

Why are we ½ mile underground to conduct the experiment?

On the earth’s surface, there is a constant flux of cosmicradiation in the form of high energy muons. These muonsinteract with the rock, etc., to produce neutrons (cosmogenic)that pollute the WIMP search region. By going deep underground, we shield the ZIP detectors from these highenergy muons.

We also use a variety of lead/polyethylene shields to blockneutrons, etc., that occur naturally in rocks.

muon vetopaddles

inner leadshield

inner polyethyleneshield

outer polyethylene shield

outer leadshield

icebox

ZIP detectors

CDMS Shielding Scheme

CDMS limit usingStanford data

EDELWEISS limit

DAMA signal region

Projected CDMS limitfor Soudan data running

Projected limitof a 1-ton cryoarray

Current World Limits for Dark Matter Searches

Shaded regions arevarious theoreticalpredictions of SUSY