Laura Baudis arrived at the University of Florida in Spring of 2004. Prior to coming here, she worked on the CDMS (that is, the Cryogenic Dark Matter Search) project during her postdoctoral stay at Stanford. Now at UF she is continuing her participation in the CDMS project as a collaborator and is also involved with the Liquid Xenon collaboration. Both projects have a very similar purpose: to detect non-baryonic dark matter called WIMPs. A WIMP is a weakly interacting massive particle. The particle most likely to be detected is the "neutralino", predicted by the super symmetric standard model, which ignores Electromagnetic Interactions.
According to cosmological models for structure formation, the luminous matter in the universe is bound to some more massive ring of dark matter. Should the dark matter of the universe consist of unidentified particles, our solar system and planet would be passing through a flux of these particles that constitute the dark halo of the Milky Way galaxy. It is this idea that encourages experiments to seek these particles on Earth.
The Liquid Xenon project represents the next generation of dark matter detection. It is similar to the CDMS project as it searches for nuclear recoils (off of Liquid Xenon in this case, not off of Germanium). The project's goal is to detect the scintillation light of particle interaction. The high mass of the Xe nucleus is favorable for WIMP scalar interactions, provided a low recoil energy threshold.
Most events will provide three signals. The first is the prompt scintillation signal detected directly by the PMTs. The last is the proportional scintillation signal from the CsI photoelectrons drifting the entire 30 cm liquid gap. These two signals are separated by exactly 150 microseconds, i.e. the maximum drift time. The difference in arrival time between the primary scintillation pulse and the proportional pulse from the electron drift measures the Zcoordinate of the event.
A small prototype being built here for proof of principle as well as for taking calibration data that will be used to compare neutron recoils to distinguish them from WIMPs. Another prototype is being constructed at Columbia University. Ultimately, a one ton liquid Xenon detector will be built in Italy.
The CDMS project is also searching for WIMPs. The actual experiment takes places deep in the mines of Soudan in Minnesota. This is actually the second phase of the experiment. The first phase, CDMSI, took place in tunnels underneath the Stanford campus. However, the proximity to the surface allowed for extensivecosmological background particles, which forced phase II to go deep underneath Earth's surface. From the collaboration website, "The CDMS experiments aim to measure the recoil energy imparted to detector nuclei through neutralino-nucleon collisions by employing sensitive phonon detection equipment coupled to arrays of cryogenic germanium and silicon crystals."
CDMSII detectors are designed with the primary functionality of detecting minute phonon signals generated by elastic collisions within a detector crystal between detector nuclei and WIMPs. The detectors themselves, known as ZIP detectors, feature thin film superconducting technology. Each 250g germanium or 100g silicon crystal provides two sets of information about interactions with incident particles. These detectors are in an array that works as follows. An incident particle collides with a nucleus in the detector generating vibrations in its crystal lattice. These vibrations are called "phonons". Phonons propagate through the crystal and some reach the surface where aluminum collector fins absorb them. Inside the aluminum, phonons convert their energy into "quasi-particles". These "quasi-particles" migrate to a tiny strip of tungsten attached to each aluminum fin. The tungsten strips are "biased" with some electrical energy that pushes them right near the brink of going through a transition between being a superconductor to being "normal". When the tungsten strips receive the energy from the "quasi-particles", they undergo transition from superconducting to normal. The tungsten strips exploit this transition as a way to sense a small amount of input of energy. This change in electrical resistance caused by the transition is amplified first by a SQUID circuit within the cryostat and then by a sophisticated series of amplifiers at room temperature. This amplified change in resistance makes the "pulse" which we observe.
The expected number of events for both experiments is really hard to say as neither the WIMP's cross-section, nor its mass is precisely known. Some believe them to have a mass in the neighborhood of 100 GeV. At least ten events per year, a worst-case scenario estimate, are predicted.
Dr. Baudis is involved with both projects because two detection materials are better than one. Once a WIMP is actually detected its properties will need to be measured, and having more than one kind of detector will broaden the possibility of information ultimately acquired.
