Fleshing out the search for WIMPs

Bertram Schwarzschild’s story “A Bubble Chamber Brings New Capabilities to the Search for WIMPs” (Physics Today, April 2008, page 22 ) is undoubtedly interesting, and the application of bubble chamber technology to searches for weakly interacting massive particles (WIMPs) by the COUPP collaboration is certainly newsworthy. However, the only “new capability” brought to bear is the spatial concentration of the active detector mass: For several years the SIMPLE ¹ and PICASSO ² research teams have used superheated liquid microdrops suspended in a gel matrix, in concentrations of 1% to 3%. The background insensitivity capabilities of these superheated suspensions are the same as a bubble chamber’s, since the underlying physics is the same. The continuous-sensitivity difficulty is why the droplet suspensions were developed in the first place; the devices are continuously sensitive for up to 100 days in the case of SIMPLE, and longer with repressurization as in PICASSO.

Achieving large active mass has simply required space and engineering of the instrumentation. The COUPP team, in achieving relatively long-term metastability of the refrigerant and long-time detector operation by rapid recompression, has succeeded not only in significantly reducing the space demand but in obtaining a sufficiently large exposure (active mass – active time) to provide new limits in the WIMP–proton coupling sector comparable with the KIMS experiment.

In contrast, the PICASSO group ran 8 devices throughout 2007 and has been running its full 32-device complement since January 2008; the SIMPLE group is similarly preparing a 30-device deployment. It is thus quite disappointing to find in the Physics Today story no mention of the current implementation of their superheated liquid capabilities or their already obtained results—which are prominently not adapted from the Science publication in the story’s figure. It’s as if the previous activity never existed.