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 of various particles e = -1 p = +1 n = 0 u = +2/3 d = -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    StrengthMediatorParticipants 1photonall nonzero 10^(-11) W +, W-, Z0all Higgs bosonall 1000gluon quarks only 10^(-40)gravitonall

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. Some possible 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 RefrigeratorIcebox Ebox Thermal stages of a dilution refrigerator: room temp.293 K(red) OVC (outer vacuum chamber) liquid N77 K(yellow) liquid He4 K(green) IVC (inner vacuum chamber) pot 1 K(light blue) still0.6 K(light blue) cold Plate 0.05 K(dark blue) mixing chamber0.01 K(black) Detectors (ZIP) (at 0.01 K stage)

Temperature (kinetic theory of thermodynamics) thermodynamic equilibrium liquid temperature = T gas pressure = P pump on gas phase liquid temp < T gas pressure < P Temperature a measure of the average energy in the random motion of a very large (~10^25) number of atoms that make the gas, liquid or solid 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 off some constituent of a crystal: 1)Lattice vibrations (phonons) 2)Electronic shells are excited and electrons are freed from being bound to a particular nucleus (ionization)

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

Picture of a ZIP detector phonon sensors Copper case is thermally connected to the 0.01 K layer of the icebox

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 search for WIMPs

Why are we ½ mile underground to conduct the experiment? On the earth’s surface, there is a constant flux of cosmic radiation in the form of high energy muons. These muons interact 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 high energy muons. We also use a variety of lead/polyethylene shields to block neutrons, etc., that occur naturally in rocks.

muon veto paddles inner lead shield inner polyethylene shield outer polyethylene shield outer lead shield icebox ZIP detectors CDMS Shielding Scheme

CDMS limit using Stanford data EDELWEISS limit DAMA signal region Projected CDMS limit for Soudan data running Projected limit of a 1-ton cryoarray Current World Limits for Dark Matter Searches Shaded regions are various theoretical predictions of SUSY