August 20, 2004 A COUPP in the making by Juan I. Collar
A COUPP in the making
A new effort to search for particle dark matter, the Chicagoland Observatory for Underground Particle Physics (COUPP), will debut at the end of this summer. A bubble chamber sensitive to Weakly Interacting Massive Particles (WIMPs) has been developed by KICP Assistant Professor Juan Collar, postdoctoral fellow Dr. Andrew Sonnenschein and their graduate students and undergraduates.
A new effort to search for particle dark matter, the Chicagoland Observatory for Underground Particle Physics (COUPP), will debut at the end of 2004. A bubble chamber sensitive to Weakly Interacting Massive Particles (WIMPs) has been developed by KICP Assistant Professor Juan Collar, postdoctoral fellow Dr. Andrew Sonnenschein and several graduate and undergraduate students (Jason Hall, Dante Nakazawa, Kevin O`Sullivan and Aza Raskin). Many of the best-motivated dark matter candidate particles are WIMPs: particles with the required characteristics appear naturally in supersymmetric extensions of the standard model of particle physics. If these particles exist, they should occasionally collide with the nuclei of ordinary atoms in the laboratory, producing a signal that would be observable. The probability that this happens is nevertheless extremely small, imposing the need for very large detectors with extraordinary background rejection abilities. Bubble chambers can be built with a unique combination of features that make them attractive for detecting WIMPs, including sensitive masses up to thousands of times greater than those available with current alternative technologies, a wide choice of optimal target nuclei, intrinsic insensitivity to most backgrounds and relatively low cost. However, the classic bubble chambers, which were built for high energy physics experiments in the 1950s-'70s, were too unstable to detect WIMPs.
The design of the COUPP detectors employs new techniques to improve stability, making a WIMP search with a bubble chamber feasible for the first time. The chamber contains two kilograms of CF3I, a fire-extinguishing liquid that can be superheated to respond to very low energy nuclear recoils, like those expected from WIMPs, while being totally insensitive to minimum ionizing environmental radiation, such as X, gamma and beta rays and muons. Bubbles produced by nuclear recoils are automatically photographed by CCD cameras. The resulting images (see Figure 1) can help further distinguish WIMPs from residual backgrounds such as neutron-induced recoils. During fall of 2004, the detector will be moved to the MINOS near detector gallery at Fermilab, where it will profit from 300 ft of rock overburden to reduce cosmic ray neutron backgrounds and the convenience of a nearby site during the testing period. COUPP collaborators at Fermilab bring in a long history of expertise in large bubble chamber development. After a few months of preliminary tests there, the chamber will be transported to the Soudan iron mine in Tower, Minnesota, where it will be installed 2400 feet underground. At this location, this first device is expected to be the most sensitive in the world for WIMPs interacting by spin-dependent scattering and to be competitive with other leading projects for detecting the spin-independent scattering mode. Since bubble chambers are easily scalable, a successful measurement at Soudan would lead naturally to the construction of much larger, more sensitive devices. In parallel with this effort, the use of small chambers as the most sensitive fast neutron detectors is to be explored in collaboration with Pacific Northwest National Laboratory.