SuperCDMS Overview Utilizing state-of-the-art cryogenic germanium detectors, the SuperCDMS (SCDMS) collaboration is searching for weakly interacting massive particles, also known as WIMPs. The discovery of these particles could resolve the dark matter problem, revolutionizing particle physics and cosmology. SuperCDMS is the successor to the CDMS II experiment, which was located deep underground in the Soudan mine in Minnesota, USA. After a brief testing period located at Soudan, SCDMS plans to be located at SNOLAB (Vale Inco Mine, Sudbury, Canada), a much deeper facility. The use of underground facilities provide shielding from cosmogenic events and as a result reduce interference of known background particles. This increases the chances of having a positive identification of a WIMP or will allow much more stringent limits to be placed on the interaction cross-section of these elusive particles.

WIMP Dark Matter While evidence for dark matter has come from many sources, such as anomalous galactic rotations, the principle evidence for dark matter being a type of exotic, non-baryonic particle comes from Big Bang Nucleosynthesis. Calculations in this model yield specific ratios of light elements in the universe, which are highly sensitive to the distribution and type of matter present. An extension of the standard model, called Supersymmetry (SUSY) offers a promising framework for the type of particle species that could fit the observed properties of dark matter. The most promising candidate particle is the lightest supersymmetric particle (LSP). This is a supersymmetric particle that all other supersymmetric particles would decay into, itself being stable. This particle is called the neutralino Χ01, which is a superposition of the fermionic superpartners of the Higgs and neutral gauge bosons. In order to be consistent with an early universe annihilation rate, leaving proper relic abundances, such a particle should have a small but measurable interaction cross-section with ordinary matter. Specifically a cross-section for interaction between a neutralino and a nucleon in ordinary matter on the order of the electroweak scale would be consistent with a meaningful cosmological role for the particle. This weak interaction cross-section, combined with a mass range of the neutralino between 10 - 100 GeV are what produce the acronym "WIMP". While the cross-section is very small, by virtue of their weak force interaction it should be possible to observe a WIMP by a direct detection experiment that observes collisions with ordinary matter.

Direct Detection of WIMPs According to models of cosmological structure formation, the luminous matter of galaxies is gravitationally bound to a more massive halo of dark matter. Should the dark matter of the universe consist of unidentified particles, our solar system and our planet would be passing through a flux of these dark matter particles which constitute the dark halo of the Milky Way galaxy. WIMP dark matter could then be detected directly as the Earth (and some detection apparatus beneath its surface) pass through our galaxy's dark matter halo. Given the weak interaction scale of the WIMP-nucleon scattering, galactic as WIMPs should deposit a measurable amount of energy in an appropriately sensitive detector apparatus. The primary mechanism this could occur through is via elastic scattering between an incident WIMP and a nucleus in the fiducial volume of some detector material. The SCDMS experiment aims to measure the recoil energy imparted to Ge nuclei through WIMP-nucleon collisions by employing sensitive phonon detection equipment. The phonon signals generated within the crystal detectors can be processed and interpreted with information about known background rates. If found, a confidently identified above-background event rate would be analyzed to determine the nature of the responsible interaction -- perhaps enabling the identification of a WIMP. Conversely, a null WIMP-nucleon scattering find can be used to greatly improve current limits on the possible WIMP interaction cross-section.

SuperCDMS Detectors SuperCDMS detectors are designed with the primary function of detecting the minute phonon signals generated within the detector crystal by elastic collisions between detector nuclei and as WIMPs. The energy deposited in a detector by an interacting WIMP may be as low as a few tens of keV. Event detection at such energy levels requires a sensitive experimental apparatus. The foremost requirement is that the detector maintained at a very low temperature to distinguish the deposited energy from the thermal energy of the detectors nuclei. The SCDMS project and associated test facilities employ He-3/He-4 dilution refrigerators which, with the appropriate cryostat apparatus (picture on the left), are able to achieve detector base temperatures as low as 10mK.

The current generation of detectors, known as iZIP (interleaved Z-sensitive Ionization Phonon) detectors feature state-of-the-art thin film superconducting technology. Each of the 600g germanium crystals (measuring 76mm in diameter and 25mm thick) provides two sets of information about interactions with incident particles. Previous ZIP detectors featured one face split into 4 phonon based sensors and the opposite face as two charge sensors to provide phonon and ionization information about an event based on the ratio of energy deposited into both types of sensors. This method was susceptible to improperly tagging low ionization events near the surfaces of of the detectors. The iZIP now features an interleaved charge ionization and phonon sensor design on both faces of the detector. The result is 8 phonon channels (4 on each face) and 4 charge channels (2 on each face). With a complex electric field near the surface that will collect electron-hole pairs, the iZIPs automatically reveal these surface events as those with a charge signal from only one face of the detector. Events further in the bulk of the detector will result in collection of electrons and holes at opposite faces and produce two ionization signals. The phonon sensors consist of an array of tiny superconducting transition edge sensors (TES) which themselves consist of microscopic strips of tungsten coupled to aluminum "fins" to collect phonon energy from the crystal.

The TES sensors function as follows: An incident particle collides with a nucleus in the detector, which sets off vibrations throughout the crystal lattice. These vibrations, which are called phonons, propagate through the crystal and some reach the surface. Once there, they are absorbed by the aluminum fins. In the aluminum fins, the phonons transfer their energy to quasi-particle Cooper pair electrons. The incident phonon energy breaks these Cooper pairs and gives the energy to the electrons. These quasi-particle electrons diffuse to the tiny strips of tungsten that are attached to the aluminum fins. A small voltage is applied across the tungsten strips in order to heat them up to the threshold of the superconducting to normal transition. When the energy carried by the quasi-particles reaches them, the tungsten strip they are pushed through the transition point and become normal. This means their electrical resistance changes dramatically with the addition of a very tiny amount of energy. The tungsten strips are thus called "transition edge sensors" since we exploit their transition from superconducting to normal as a way to sense a small input of energy. The change in the TES resitance causes a small change in the current flowing through them. This current change is amplified first by a SQUID circuit within the cryostat itself and then by a sophisticated series of amplifiers at room temperature. This amplified change in current makes the "pulse" which we observe.

The tungsten TES array (and accompanying aluminum fins) on the phonon-detecting side of the detector is divided into 8 "channels" (4 on top and 4 on the bottom), each containing over 1000 tungsten sensors. The picture on the left displays a one sided view of 4 channels; both top and bottom views look like said picture.

Physics Goals The SCDMS experiment seeks to combine high fiducial detector mass and extensive run time with reduced backgrounds to advance WIMP dark matter knowledge significantly. With advanced new detector technology we will improve upon the limits set in CDMS II and rule out conflicting annual modulation signals seen in DAMA and CoGeNT collaborations.

Education and Outreach
(Click on the image above enlarge our education poster, which is on display at the Soudan mine!)

Other Links Sadoulet Group - University of California, Berkeley - DMpages - Explore the Science of Dark Matter

This work is supported by the National Science Foundation and the Department of Energy