Dark Matter Search With The First Year Of Data From The DEAP-3600 Experiment

Abstract

Dark Matter is a fundamental building block and is essential for the universe to be understood on many scales. Currently, the top candidate for Dark Matter is a weakly interacting massive particle (WIMP). The DEAP-3600 experiment was built to search for rare interactions between baryonic matter and a WIMP. With 3257 +/- 112 kg of liquid Argon (LAr), DEAP-3600 searches for elastic collisions between WIMPs and Ar nuclei. Particle interactions with Ar create scintillation light that is measured with 255 Photomultiplier Tubes (PMTs). The PMTs are coupled to the detector through 0.5 m long light guides (LGs). To optimize the light collection efficiency of the detector and maximize the structural integrity of the detector, the LGs were bonded directly to the acrylic vessel (AV). To perform the LG bonding special equipment and techniques were developed in order to properly attach each of the LGs in such a way that they withstand the thermo-stresses from the cryogenic cooldown of the detector. An overview of the LG bonding development and execution is discussed. The signal from the PMTs are recorded with digitizers. The PMT pulses are analyzed with a derivative pulse finding method capable of handling a large dynamic range of pulse rates and are fit with a resolution of ~0.1 ns. A data reduction method is used to remove the digitized pulse traces from single photo-electrons. The removed pulse traces are replaced with summary information about the pulse providing ~45% reduction in data file size. The optical characteristics of the detector have been studied and the optical parameters of the detector simulation have been tuned to match. The optical parameters for liquid argon (LAr) are updated and the optical models for the wavelength shifter and LG reflectors are optimized to improve the constancy between data and Monte Carlo. The tuned optics allows improved understanding of the detector and reduced radial bias in the event position reconstruction. Overlapping 39Ar-on-39Ar and 39Ar-on-Cherenkov events (pile-up) are removed from data using several specially developed techniques. Using a Monte Carlo to generate pile-up events from two data events, the methods are shown to be sufficient at removing pile-up events with no leakage of pile-up events entering the WIMP region of interest. The acceptance to single 39Ar decays and 40Ar recoils is measured using simulated events from the RAT detector simulation. An analysis is performed using 2.05 +/- 0.07 tonne-years of data, a limit on the WIMP-nucleon cross-section was set at 9.24 x 10^-45 cm^2 for a 100 GeV WIMP(90% C.L.). This result is strongly limited by the presence of a large, currently unknown background. The background is inconsistent with neutron leakage as well as radioactivity at the edges of the LAr volume.