Very few confirmed, experimentally-driven problems remain in particle physics. The solar neutrino problem, in which several experimental measurements disagreed strongly with the Standard Model of particle physics (as well as with the Standard Solar Model), is the most recent such problem to have been resolved. The existence and nature of dark matter is among those problems left: we know, based on an extensive array of systematically different astronomical observations, that there is something that behaves like bulk, gravitationally-interacting matter, and which makes up roughly 20% of the energy density of the Universe. While we do not know what the solution to this problem will be—it could, after all, be a theoretical solution or perhaps even an astrophysical solution, as unlikely as that may be at this point—it is nonetheless a problem for which a solution must exist.

Among the suggestions for the nature of dark matter are that it is composed of Weakly
Interacting Massive Particles (WIMPs), and a popular candidate for such a particle is provided by the lightest supersymmetric particle (LSP). The expected weak interaction cross section for the LSP may be as high as 10−45cm2 for a WIMP weighing 100 GeV, which means the direct detection—or perhaps even the exclusion—of a SUSY-inspired dark matter WIMP is possible by terrestrial experiments. The discovery of WIMPs, in conjunction with a discovery of supersymmetry at the Large Hadron Collider, would likely be among the great intellectual triumphs of the past century, and would tie together the fields of astrophysics, cosmology, particle physics and nuclear physics more directly than ever. The direct detection of dark matter was among the highest priorities in particle physics, as detailed in the EPP2010 report.
There are many approaches to dark matter detection: beautiful solid state devices like those used by the CDMS experiment, detectors using inorganic scintillators, and, more recently, liquid noble gas detectors like those of the XENON-10 experiment. The world’s best limits on the interaction cross section for WIMPs (_ few×10−44cm2) are held jointly by the CDMS and XENON-10 experiments although a claim of a discovery by the DAMA/LIBRA experiment has recently been revived based on new data. The continued tension between the DAMA/LIBRA measurements and the other experiments, and the great importance of the dark matter question, argue strongly for the continued support of multiple approaches to the question.

The DEAP/CLEAN experiment is a planned liquid noble gas detector, which will be capable of using both liquid argon and liquid neon as the target and detection medium. What makes DEAP/CLEAN unique is that, unlike other liquid noble gas experiments that require both liquid and gas phases to detect scintillation light and ionization, DEAP/CLEAN is a purely liquid-phase detector. The detection of the expected nuclear recoils from a WIMP interaction is done exclusively through scintillation light, and therefore no large electric fields (like those used in the ionization-based liquid noble gas detectors) are needed. Backgrounds from electrons and -rays can be eliminated through the very different timing profiles of nuclear recoils and electrons in either argon or neon. Nuclear recoils create more singlet state dimers than triplet states, and hence create more ‘prompt’ light than do electron events. The relative timing of the scintillation light for the two states in argon is enormously different—nanoseconds for the singlet, 1-2μs for the triplet. The consequence of this behavior is that the ratio of prompt/total
light (‘fprompt’) for each event can be used as the discriminating quantity. In the case of argon, the pulse shape discrimination (PSD) may be as large as 1 part in 1011.
The advantage of the single-phase approach is first, that the detector can be very simple and therefore easy to characterize, simulate, and understand. Second, the simplicity means that it can be built very large and at very low cost relative to other dark matter detectors. The large size allows the removal of external backgrounds such as neutrons from the PMTs or radon daughters at the edge of the container volume. The overall reduction of backgrounds is therefore accomplished with two ‘orthogonal’ cuts: fiducialization through reconstruction of the event location, and the requirement of high (recoil-like) fprompt. For a one-ton fiducial volume detector, a PSD level of 10−10 and a reconstruction resolution of roughly 12 cm would lead to a sensitivity to WIMPs with interaction cross sections of a few times 10−46cm2, pushing into the predicted regime for an LSP. One final advantage of this approach is that the same detector technology—run with liquid neon—can be scaled up further to allow detection of solar neutrinos down to the pp chain, one of the three priorities listed in the APS neutrino study the ‘Neutrino Matrix’.

The basic design of a single-phase detector resembles the now almost ubiquitous configuration of many low energy non-accelerator experiments: a spherical active volume surrounded by photomultiplier tubes (PMTs). Because the scintillation light of both liquid argon and liquid neon is in the hard ultraviolet (80 nm for liquid neon, 125 nm for argon), a wavelength shifter is needed to down-convert the UV to visible wavelengths, and tetraphenyl butadiene (TPB) has been shown to work well, re-emitting the light with high efficiency at 440 nm. The heavy reliance on scintillation light (for both the pulse shape discrimination and for reconstruction) means that the detector must have high effective photocathode coverage, which can be accomplished by using a large number of PMTs as well as coating uncovered areas with diffuse reflector. The DEAP/CLEAN approach was one of three liquid noble gas experiments endorsed by the Dark Matter Scientific Assessment Group (DMSAG).

The DEAP/CLEAN Collaboration formed out of the merger of the US CLEAN Collaboration and the Canadian DEAP Collaboration, each of which were pursuing similar single-phase approaches. The collaboration now includes in the U.S. Los Alamos National Laboratory, Yale University, Boston University, NIST, the University of North Carolina, the University of New Mexico, MIT, and the University of Pennsylvania, and, in Canada, Queens University, Carleton University, SNOLab, and the University of Alberta.