Active Shielding For Future Large Scale Dark Matter Experiments PDF Download

The nature of non-baryonic dark matter is one of the most intriguing questions for particle physics at the start of the 21st century. There is ample evidence for its existence, but almost nothing is known of its properties. WIMPs are a very appealing candidate particle and several experimental campaigns are underway around the world to search for these particles via the nuclear recoils that they should induce. The COUPP series of bubble chambers has played a significant role in the WIMP search. Through a sequence of detectors of increasing size, a number of R & D issues have arisen and been solved, and the technology has now been advanced to the point where the construction of large chambers requires a modest research effort, some development, but mostly just engineering. It is within this context that we propose to build the next COUPP detector - COUPP-500, a ton scale device to be built over the next three years at Fermilab and then deployed deep underground at SNOLAB. The primary advantages of the COUPP approach over other technologies are: (1) The ability to reject electron and gamma backgrounds by arranging the chamber thermodynamics such that these particles do not even trigger the detector. (2) The ability to suppress neutron backgrounds by having the radioactively impure detection elements far from the active volume and by using the self-shielding of a large device and the high granularity to identify multiple bubbles. (3) The ability to build large chambers cheaply and with a choice of target fluids. (4) The ability to increase the size of the chambers without changing the size or complexity of the data acquisition. (5) Sensitivity to spin-dependent and spin-independent WIMP couplings. These key advantages should enable the goal of one background event in a ton-year of exposure to be achieved. The conceptual design of COUPP-500 is scaled from the preceding devices. In many cases all that is needed is a simple scaling up of components previously used. Calibration and R & D are still needed on some aspects of the system. We know we have the ability to distinguish alpha-induced events from nuclear recoils, but we do not yet know whether the combination of material purity and rejection are good enough to run for a year with no alpha background. We also need to have more detailed measurements of the detector threshold and a better understanding of its high gamma rejection. In addition, there are important checks to make on the longevity of the detector components in the hydraulic fluid and on the chemistry of the active fluid. The 2009 PASAG report explicitly supported the construction of the COUPP-500 device in all funding scenarios. The NSF has shown similar enthusiasm. It awarded one of its DUSEL S4 grants to assist in the engineering needed to build COUPP-500. The currently estimated cost of COUPP-500 is $8M, about half the $15M-$20M price tag expected by the PASAG report for a next generation dark matter search experiment. The COUPP-500 device will have a spin independent WIMP-nucleus cross-section sensitivity of 6 x 10−47 cm2 after a background-free year of running. This device should then provide the benchmark against which all other WIMP searches are measured.