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Results from the DarkSide-50 Dark Matter Experiment

  • Author(s): Fan, Alden
  • Advisor(s): Wang, Hanguo
  • et al.
Abstract

While there is tremendous astrophysical and cosmological evidence for dark matter, its precise nature is one of the most significant open questions in modern physics. Weakly interacting massive particles (WIMPs) are a particularly compelling class of dark matter candidates with masses of the order 100 GeV and couplings to ordinary matter at the weak scale. Direct detection experiments are aiming to observe the low energy (<100 keV) scattering of dark matter off normal matter. With the liquid noble technology leading the way in WIMP sensitivity, no conclusive signals have been observed yet. The DarkSide experiment is looking for WIMP dark matter using a liquid argon target in a dual-phase time projection chamber located deep underground at Gran Sasso National Laboratory (LNGS) in Italy. Currently filled with argon obtained from underground sources, which is greatly reduced in radioactive Ar-39, DarkSide-50 recently made the most sensitive measurement of the Ar-39 activity in underground argon and used it to set the strongest WIMP dark matter limit using liquid argon to date.

This work describes the full chain of analysis used to produce the recent dark matter limit, from reconstruction of raw data to evaluation of the final exclusion curve. The DarkSide-50 apparatus is described in detail, followed by discussion of the low level reconstruction algorithms. The algorithms are then used to arrive at three broad analysis results: The electroluminescence signals in DarkSide-50 are used to perform a precision measurement of longitudinal electron diffusion in liquid argon. A search is performed on the underground argon data to identify the delayed coincidence signature of Kr-85 decays to the Rb-85m state, a crucial ingredient in the measurement of the Ar-39 activity in the underground argon. Finally, a full description of the WIMP search is given, including development of cuts, efficiencies, energy scale, and exclusion curve in the WIMP mass vs. spin-independent WIMP-nucleon scattering cross section plane.

This work was supervised by Hanguo Wang and was completed in collaboration with members of the DarkSide collaboration.

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