The existence of dark matter is known from gravitational effects, but its nature remains a deep mystery. One possibility motivated by other considerations in elementary particle physics is that dark matter consists of undiscovered elementary particles; Weakly Interacting Massive Particles (WIMPs) are one possibility. Evidence for WIMPs that could constitute dark matter may come from experiments at the Large Hadron Collider at CERN or from sensitive astronomical instruments that detect radiation produced by WIMP-WIMP annihilations in galaxy halos. The orbital motion of the WIMPs composing the dark matter halo pervading the galaxy should result in WIMP-nuclear collisions of sufficient energy to be observable by sensitive laboratory apparatus.
This award will provide funding for research and development steps to support DarkSide-G2, a second-generation ("G2") direct WIMP search using a liquid argon Time Projection Chamber (TPC) with an active mass of 3.3 tons. Liquid argon is a promising 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. This is one of the most powerful background rejection factors among all dark matter detectors, and when combined with the measurement of ionization, background rejection is even further enhanced. The performance of a ton-scale or larger liquid argon TPC would be limited by the high rate of 39-Ar beta decays if atmospheric argon was used. DarkSide-G2 will use argon collected from underground sources which has been shown to have 39-Ar content lower than atmospheric argon by at least a factor of 150. These funds will allow the groups to carry out the detailed mechanical design work necessary to ensure the functionality of the detector, develop necessary high voltage and data acquisition elements, and develop and identify the extremely radiopure detector components and techniques necessary to maximize the sensitivity of the experiment.
Broader impacts: This activity will advance the development of astroparticle physics and its scientific and educational mission in a variety of ways: (1) it will offer a continuing opportunity for the training of students, who will have a chance to contribute a cutting-edge project in fundamental science and advanced engineering; (2) it will benefit society by developing techniques that could find application in areas ranging from national security to medical imaging; and (3) it will support continued development of successful E&O programs such as the LNGS-South Dakota-Princeton summer school for high school students.
It is generally inferred from astronomical measurements that Dark Matter comprises 17% of the energy-density of the universe and is responsible for the galactic structures which are visable today. The nature of Dark Matter is unknown other than it is non-luminous and very weakly interacting. The DarkSide G2 experiment was funded to develop the instrumentation to detect a collision of a Dark Matter particle as our Earth moves through the Dark Matter field of the Milky Way. These collisions produce ionization in the detector, which in DarkSide is a liquid Argon cryostat operating at -186 deg. C. Thus detection occurs at cryogebic temperatures, and signals from an event are very small. The University of Houston, within the DarkSide collaboration, developed a cryogenic preamplifier to transmitt signals from the sensor in the liquid Argon to the processing electronics some 5 meters distant. In addition, events are rare so that extreme measures need to be taken to mitigate and identify backgrounds. Houston studied by computer simulations potential backgrounds, and developed the electronic systems by which they could be mitigated. All systems and components have been tested in the first stage DarkSide50 detector which is now acquiring data at the LNGS laboratory in Italy. The figure shows the detection systems which include the detector cryostat inside a background suppression liquid scintillator and water veto. In summary, the research developed an amplifier operational at -186 deg C, and studied the removal, identification, and mitigation of low level cosmic and naturally occuring radioactivity.
