UC Berkeley

Dark matter makes up most of the mass of the universe, and yet the true nature of this mysterious substance remains unknown. The LUX experiment uses a detector consisting of target xenon nuclei to search for Weakly Interacting Massive Particles, a promising dark matter candidate. The LUX detector is a 370 kg dual-phase xenon time projection chamber. Incident radiation interacts with xenon atoms, resulting in a recoil nucleus or electron that causes both ionization and excitation of Xe along the recoil track. De-excitation of Xe results in the emission of 175 nm scintillation light, and the properties of this scintillation can be used to determine the nature of the incident radiation - possibly identifying WIMP dark matter.

LUX has placed world-leading limits on the WIMP-nucleon interaction cross section. The rarity of such interactions requires a thorough understanding of backgrounds in order to implement appropriate background-reducing measures and an effective event-rejection methodology. The LUX water tank surrounds the xenon detector and serves as an active shield, reducing background radiation but also identifying high energy cosmic ray muons that can lead to the production of a highly undesirable neutron background. Since signals created by neutrons may appear very similar to those of WIMPs, care must be taken to not only reduce neutron backgrounds as much as possible, but to also understand any neutron backgrounds that may be present.

The goal of this dissertation was to develop a method to identify cosmic ray muons that traverse both the LUX Xe detector and water tank, and to measure a muon flux at the LUX detector depth. The xenon gas-liquid interface served as a fiducial surface through which muon flux was measured. Simultaneous signals between Xe and water detectors were analyzed, and xenon pulse shapes were used to determine the energy and track geometry of the interaction. Muons passing the Xe liquid surface exhibited a particular signature in their pulse shape; this signature was used to identify through-going muons and calculate muon flux.

In a previous work, muon flux measurements were taken at sites above the LUX detector, and muon transport models were used to predict vertical muon flux as 4.40 x 10^-9 muons s^-1 cm^-2 at the LUX detector depth. In this work, a flux of 4.60 +/- 0.33_(stat) x 10^-9 muons s^-1 cm^-2 is measured through the LUX water tank and xenon detector liquid surface. The resulting neutrons expected to be seen during the full LUX exposure is ~1 x 10^-1. While expected the background contribution from muon-induced neutrons poses little problem for the LUX WIMP search, an understanding of this background becomes increasingly important as target volumes grow for future generation experiments.