COUPP-250 kg. A Large-Mass Bubble Chamber Search for Weakly Interacting Massive Particles

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The possibility of using room-temperature moderately superheated liquids in the bulk (i.e., bubble chambers) for rare event searches has been demonstrated within the COUPP project (Chicagoland Observatory for Underground Particle Physics). The prototype chambers developed display a stable behavior and by careful selection of operating temperature and pressure, the target liquid can be made sensitive to low-energy nuclear recoils like those expected from Weakly Interacting Massive Particles (WIMPs), while remaining insensitive to minimum ionizing particles: an unparalleled gamma rejection factor has been demonstrated for COUPP chambers. In this proposal, the PI requests funds for support towards a 250 kg total mass in the form of five small 50 kg chambers, to be deployed in one of the underground laboratories during the period covered by this proposal. The target liquid employed, CF3I, a safe industrial refrigerant commonly used as a fire extinguisher, is simultaneously maximally sensitive to both spin-dependent and -independent Lightest Supersymmetric Partner couplings. The proposed detector ensemble features a spin-independent potential sensitivity equivalent to ~550 kg of Germanium, and can simultaneously improve the present sensitivity to spin-dependent couplings to the point that supersymmetric models will be probed along both fronts. COUPP aims not at a mere improvement of present-day WIMP sensitivity, but also at implementing a variety of targets and methods able to provide convincing evidence that an eventual signal arises from WIMPs and not some irreducible background. This research will continue to provide multiple opportunities for undergraduate involvement. The use of stable, structurally simple mini-bubble chambers as background-insensitive, high-efficiency fast neutron monitors for portal monitoring of cargo containers is also being developed.