It is now generally accepted in the scientific community that roughly 85% of the matter in the universe is in a form that neither emits nor absorbs electromagnetic radiation. Multiple lines of evidence from cosmic microwave background probes, measurements of galaxy cluster and galaxy rotation curves, strong and weak gravitational lensing and big bang nucleosynthesis all point toward a cosmological concordance model containing cold dark matter particles as the best explanation for the universe we see. Alternative theories involving modifications to Einstein's theory of gravity have not been able to explain the observations across all scales. A compelling candidate for dark matter is Weakly Interacting Massive Particles (WIMPs) that could be directly detected as they scatter from massive, ultra-pure detector targets operating deep beneath the Earth's surface.
The challenge to realizing sensitive dark matter detectors is in separating ubiquitous backgrounds from the nuclear-recoil events characteristic of the WIMP signature. Indeed, we face the daunting task of separating a single event in a tonne of target material or more following a year's exposure. Noble liquid detectors exploiting liquid xenon or liquid argon hold great promise in realizing this intimidating goal. LANL scientists are spearheading a novel approach, dubbed CLEAN for Cryogenic Low-Energy Astrophysics with Noble Liquids, to the direct detection of dark matter using the unique capabilities of liquid argon for unprecedented discrimination between electromagnetic backgrounds (electrons and gamma rays) and nuclear-recoil events (ref.). CLEAN is unique in its ability to exchange the liquid argon target with liquid neon in the same detector, offering potential to the discovery of WIMP dark matter in a "beam-on, beam-off" approach and further expanding its scientific portfolio to the detection of low-energy neutrinos.
MiniCLEAN (see Figs.1&2), a first-generation experiment with a target (fiducial) mass of 500 kg (150 kg), will be the first "single-phase" LAr detector capable of 3-D event reconstruction . The ability to inject a radioactive spike of 39Ar into the MiniCLEAN detector provides a unique opportunity to study background discrimination at the unprecedented level of parts in 1010. The data from MiniCLEAN will inform the ultimate PSD capability of next generation experiments like DEAP-3600 and DarkSide-G2 and provide the bedrock for design of a massive detector capable of extending the reach for WIMP dark matter by several orders of magnitude. The MiniCLEAN detector is presently being assembled in the Cube-Hall at SNOLAB (see Fig.3) and includes collaborators from the U.S., the United Kingdom and Canada.
 M.G. Boulay and A. Hime, A Technique for Direct Detection of Weakly Interacting Massive Particles Using Scintillation Time Discrimination in Liquid Argon, Astroparticle Physics 25, 179-182 (2006).
 A. Hime, The MiniCLEAN Dark Matter Experiment, Proceedings of the DPF-2011 Conference, providence, RI (2011); arXiv:1110.1005.