Intellectual Merit: One of the great mysteries in astrophysics is the nature of dark matter, which is currently only detected indirectly through its effects on the structuring of galaxies, clusters of galaxies and larger scale structures in the universe. Generic extensions of the standard model of subatomic particles predict the existence of Weakly Interacting Massive Particles (WIMPS), which are excellent candidates for the dark matter. These are expected to interact with ordinary matter rarely and thus be extremely difficult to detect. The COUPP and PICASSO groups are developing new large dark matter detectors derived from the well-known bubble chamber technique. Charged particles traversing the superheated fluid in bubble chambers deposit heat, which creates small bubbles of gas. The bubbles can be photographed before pressurization revealing the paths of these particles. Other non-dark matter triggers can also lead to runaway boiling. These have limited classical chambers to remain in a superheated state for only a short time. The COUPP experiment has paid special attention to material selection and manufacturing techniques to remove roughness and radiocontamination to allow the superheated state to be long lived. The PICASSO experiment suspends superheated fluid droplets in a gel whose surfaces are so smooth that the droplets can stay in a superheated state indefinitely. Both experiments have made the superheated state sufficiently long lived to search for dark matter.
This award will allow the PI to continue his work on the COUPP and PICASSO experiments. The sound sensors produced by the Indiana University group have been used for rejecting cosmic ray induced events and distinguishing the radioactive signals for moderately superheated fluids. The PI and his students have built equipment to test the sound sensors under realistic conditions mimicking those in the detectors and to measure changes in their sensitivity as a function of age. The group has also taken on the task of measuring contaminants in the fluid target and is now prepared to routinely assay the fluid.
Broader impacts: Enhancing the education of undergraduates is an integral part of this program. Undergraduates, high school teachers and students have participated in design, construction and characterization of acoustic transducers, measurement of radioactivity, and other projects. In addition, the PI and his group has provided acoustic sensors to other scientific groups to assist in other experiments.
