Projects: Chicagoland Observatory for Underground Particle Physics (COUPP)

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COUPP uses stable room-temperature bubble chambers to search for Weakly Interacting Massive Particles (WIMPs), which might compose a significant fraction of the galactic Dark Matter. The superheated refrigerant used, CF3I, is a fire-extinguishing agent and an optimal target for both spin-dependent and spin-independent WIMP couplings. At the moderate degrees of superheat necessary to detect low-energy nuclear recoils like those expected from WIMPs, this fluid exhibits a measured intrinsic rejection of minimum-ionizing backgrounds better than 1010. The metastable superheated state is however sensitive to alpha-recoils, resulting in a requirement to reach ultra-trace levels of elements in the U and Th decay chains within the active volume of the chambers. The eventual goal is to match the radio-purity in U and Th of modern large neutrino detectors.

COUPP presently operates a 2 kg chamber at the ~300 m.w.e. depth of the NuMI tunnel at Fermilab as a test beam precursor of this project (T-945). During T-945 valuable information has been obtained on the presence and control of radon-induced backgrounds. More than 250 kg-days of exposure were collected, demonstrating a stable and reproducible operation of the chamber. The dataset obtained already leads to a very competitive sensitivity to spin-dependent couplings, even in the presence of identifiable backgrounds. Measures against these backgrounds are being implemented at the present time (January 2007). Successful calibrations of the device with gamma and neutron sources were also performed during T-945.

The short-term goals for COUPP are to reduce the alpha-recoil backgrounds in the 2kg chamber to a level of less than one event per kg per day, and to apply the upgrades tested on it to larger chambers currently under construction. A pion beam calibration able to separate the response to iodine and fluorine recoils is planned, using a dedicated chamber. With the addition of a high-efficiency muon veto presently being commissioned, it is in principle possible to reach improved sensitivities to both WIMP couplings in the present Fermilab location. The collaboration is constructing a larger device, totaling up to 60 kg of CF3I. This detector will undergo tests at shallow depth in the NuMI tunnel, and will be prepared for deployment in a suitable deep-underground location. Devices of this mass will be able to exploit a number of signatures characteristic of WIMP-induced recoils, leading to a diminished sensitivity to alpha and neutron recoils and to a much-improved WIMP sensitivity. We have been charged to assess the scalability of this technique to the deep-underground installation of a target mass of order one ton, using a number of different refrigerant targets immersed in a suitable shielding and leading to an exhaustive exploration of supersymmetric WIMP models.