The COUPP program puts a new twist on an old technology, bubble chambers, to search for the dark matter that physicists think constitutes the majority of the matter in the Universe. Recently, COUPP has been able to discriminate between potential dark matter signals and one of the most troublesome backgrounds, alpha decays, by listening to the sound of bubble formation – alphas are louder. Results showing this effect and setting new limits on dark matter interactions were published in January (PRL, 106, 021303). The COUPP-4 chamber is now operating in an underground lab in Ontario called SNOLAB, and this new run is demonstrating a rejection of alpha backgrounds of better than 98%. The figure above shows this rejection – the “AP” is a measure of the “loudness” of a bubble, with the blue population representing what a dark matter particle would sound like and the red population representing largely alpha events. When the current run finishes, the COUPP program will take another big step forward in sensitivity to dark matter.
-Jeter Hall