Disque Hall 919, S. 32nd Street and Chestnut Street, Philadelphia, PA 19104
Russell Neilson, PhD, postdoctoral associate, University of Chicago
Astrophysical and cosmological observations have now measured with some precision the fraction of dark matter in the universe. However, the particle nature of dark matter remains unknown, and is the subject of an intense world-wide effort to observe interactions of dark matter particles in laboratory experiments. The PICO collaboration, formed by the recent merger of the PICASSO and COUPP collaborations, uses bubble chambers filled with various refrigerants to search for nuclear recoils from WIMP dark matter particles. The bubble chambers are operated in a moderately superheated state providing superb rejection of the dominant gamma background at better than the 10-10 level. Two target fluids have been used until now, CF3I, with sensitivity to both spin-dependent (SD) and spin-independent (SI) interactions, and C3F8, with improved sensitivity to SD interactions and low-mass WIMPs. The first bubble chamber filled with CF3I, COUPP-4kg, has completed two physics runs deep underground at SNOLAB in Canada, demonstrating rejection of alpha-decay backgrounds by acoustic discrimination and providing competitive limits on SD interactions. Two new bubble chambers are now operating at SNOLAB, COUPP-60kg filled with CF3I and PICO-2L filled with 2 liters of C3F8. The next generation in the PICO campaign, PICO-250L, is currently being designed.