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. Axions and Weakly Interacting Massive Particles (WIMPs) are two possibilities. The thermal motion of the WIMPS comprising the dark matter halo surrounding the galaxy and the earth should result in WIMP-nuclear collisions of sufficient energy to be observable by sensitive laboratory apparatus.
This award is to develop and deploy a small liquid argon detector that has high sensitivity for direct detection of WIMP collisions. This detector, DarkSide-50, builds on past experience and introduces innovative features that will allow it to operate in a background-free mode and thereby achieve a significant science result in spite of its relatively small size. At the same time, this detector will serve as a prototype for a future multi-ton detector. The main innovations being introduced: - Underground argon depleted in radioactive 39Ar; - Low background, high-quantum-efficiency QUPID photo-detectors; and - A compact high-efficiency external veto for neutrons. These innovations, together with the powerful two-parameter background rejection features of argon will result in a detector of unprecedented background-free performance. DarkSide-50 will either detect WIMP dark matter or exclude a substantial fraction of the favored parameter space. DarkSide-50 will benefit from the facility for extraction and refinement of depleted argon, independently funded by the NSF through a grant to Princeton University.
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 an excellent opportunity for the training of students, who will have a chance to contribute to the success of 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; (3) it will support continued development of successful E&O programs such as the Princeton-Abruzzo-South Dakota summer school for high school students.
The major goal of the project was construction and commissioning of the first generation dark matter detector DarkSide-50, designed to perform a dark matter search with underground argon as a target. Underground argon collected by the DarkSide collaboration in Colorado had been shown to have very reduced values of the radioactive 39Ar relative to atmospheric argon, and is an ideal target for dark matter searches. DarkSide-50 is the first dark matter experiment designed to operate within a 4π neutron veto (a 30-tonne liquid scintillator detector) and within a 4π muon veto (a 1,000-tonne water Cerenkov detector). With support from this award: We built and commissioned the SCENE detector, a small, 3"×3", LAr-TPC operated at the center of a set of coincidence liquid scintillator detectors, apt to collect calibration data on the response of liquid argon detector to nuclear recoils. We operated the SCENE experiment in three different campaigns, exposing it to a pulsed, monochrormatic neutron beam at the Notre Dame Nuclear Science Laboratory, and collecting data that permitted to obtain calibration data on the response of liquid argon detector to nuclear recoils. With the SCENE detector, we measured the scintillation and ionization yield of recoiling nuclei in liquid argon as a function of applied electric field. Liquid scintillation counters were arranged to detect and identify neutrons scattered in the TPC and to select the energy of the recoiling nuclei. We reported measurements of the scintillation and ionization yields for nuclear recoils with energies from 10.3 to 57.2 keV and for applied electric fields from 0 to 1000 V/cm. We also reported the observation of an anti-correlation between scintillation and ionization from nuclear recoils, which is similar to the anti-correlation between scintillation and ionization from electron recoils. We completed construction and commissioning of the DarkSide-50 LAr-TPC, of the 30-tonne liquid scintillator neutron veto, and of the 1,000-tonne water Cherenkov cosmic rays veto. Upon completion of commissioning, we discovered that one of the two components of the scintillator, TriMethylBorate (TMB), was contaminated with high levels of the cosmogenic 14C since the chemical was produced from methanol. We upgraded our processing plants to make possible the separation of TMB from the other component of the scintillator, PseudoCumene. We removed completley the TMB replacing it with PC. We procured a new batch of TMB produced from petroleum and not contaminated with high levels of 14C and filed the new TMB in the liquid scintillator detector, completing the refurbishment of said detector. We performed a first dark matter search with an exposure of (1422±67) kg×d with an atmospheric argon fill of DarkSide-50, registering no background events. The result was the most sensitive dark matter search performed with an argon target, corresponding to a 90% C.L. upper limit on the WIMP-nucleon spin-independent cross section of 6.1×10−44 cm2 for a WIMP mass of 100 GeV/c2. We completed the procurement of a 150 kg batch of argon from underground wells, protected by the cosmic rays which produce the radioactive 39Ar polluting argon in the atmosphere.
