The existence of dark matter is inferred from gravitational effects, but its nature remains a deep mystery. One possibility, motivated by considerations in elementary particle physics, is that dark matter consists of the hypothesized Weakly Interacting Massive Particles (WIMPs). It should be possible to detect WIMPs directly, as the orbital motion of the WIMPs composing the dark matter halo pervading the galaxy should result in WIMP-nucleus collisions of sufficient energy to be observable in the laboratory. This collaboration utilizes funds to commission and operate DarkSide-50, a WIMP search using a Liquid Argon Time Projection Chamber (LAr-TPC) with an active mass of 50 kg. DarkSide-50 will use argon extracted from underground sources (Underground Argon, UAr), which this group has shown to have an Ar-39 content lower, by at least a factor of 150, than atmospheric argon. Ar-39 is one of the main sources of background in the experiment.

A significant by-product of this group's research has been developing techniques that could find application in areas ranging from national security to medical imaging. This arises from the extraction of rare noble gases (Argon and Helium) from underground sources, and possible applications include the detection of underground nuclear tests and in Environmental Science where Argon is used as a detection media in ultra-low level proportional counter measurements for an environmental radio tracer for hydrologic transport.

Liquid argon is an attractive medium for WIMP detection due to its efficient conversion of energy from WIMP-induced nuclear recoils into both ionization and scintillation. The argon scintillation time profile ("pulse shape") depends on the type of ionizing particle, providing particle discrimination that can be used to suppress background. Pulse shape discrimination in argon provides one of the most powerful background rejections among all dark matter technologies; when combined with the measurement of ionization, the background rejection is further enhanced.

The proposed activity will advance the development of astroparticle physics and its scientific and educational mission by: (1) offering a continuing excellent opportunity for the training of students, who will contribute to the success of a cutting-edge project; (2) developing techniques that could find application in several areas of benefit to society; and (3) supporting a new education and outreach program, designed to succeed the highly successful Laboratori Nazionali del Gran Sasso-South Dakota-Princeton summer school.

National Science Foundation (NSF)
Division of Physics (PHY)
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Jonathan Whitmore
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Princeton University
United States
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