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Optimization of Signal Versus Background in Liquid Xe Detectors Abstract of “Measurement and Analysis of WIMP Detection Backgrounds, and Characteri- zation and Performance of the Large Underground Xenon Dark Matter Search Experiment” by David Charles Malling, Ph.D., Brown University, May 2014. The dominant component of matter in the universe, referred to as dark matter, cannot be explained by the standard model of particle physics. A leading candidate for dark matter is the weakly interacting massive particle (WIMP). The WIMP theory is well motivated by several extensions of the standard model, and has been tested directly in the laboratory over the last 30 years. The Large Underground Xenon (LUX) experiment seeks to identify the signatures of WIMP interactions with baryonic matter. LUX measures particle interactions at the keV level by the detection of single-photon signals in a 250 kg Xe target mass. The LUX detector inner fiducial region is the lowest measured background particle detector in the world at 3 1 1 1 keV energies, with background rates at the level of 10− counts keV− kg− day− . Low background rates are essential for identifying WIMP interactions in the detector, which 1 1 occur with frequencies <1kg− (5 years)− . LUX completed its first low-background science run at the Sanford Underground Re- search Facility in August 2013. WIMP search results are reported from an 85 live day anal- ysis. The experiment places the tightest constraint on WIMP spin-independent interaction cross-sections to date. Results from LUX also exclude several potential dark matter signal claims from other direct detection experiments, for WIMP masses in the range 6–15 GeV. The LUX result is the product of low detector background rates and high photon signal collection efficiency. Analysis of the first WIMP search data has provided direct characterization of low- energy backgrounds in LUX. Measured background rates were found to be in agreement with expectations based on models of detector material radioactivity. The data also provides the first measurements of intrinsic radioactive contaminants in the detector, including short- lived cosmogenic Xe radioisotopes. I will describe the LUX background model, detailing work which was used in the design and early performance projections of the LUX detector, as well as background measurements which constrain and support the model. Measurement and Analysis of WIMP Detection Backgrounds, and Characterization and Performance of the Large Underground Xenon Dark Matter Search Experiment by David Charles Malling Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Program in Physics at Brown University Providence, Rhode Island May 2014 Copyright © 2013 David Charles Malling This dissertation by David Charles Malling is accepted in its present form by the Physics Department as satisfying the dissertation requirement for the degree of Doctor of Philosophy. Date: Richard Gaitskell, Advisor Recommended to the Graduate Council Date: David Cutts, Reader Date: Savvas Koushiappas, Reader Approved by the Graduate Council Date: Peter Weber, Dean of the Graduate School iii Curriculum Vitae 1984 Born in Syracuse, NY (USA) 2007 B.S. Engineering Physics. Syracuse University, Syracuse NY (USA) 2010 M.S. Physics. Brown University, Providence RI (USA) Appointments and Teaching 2007- Research Assistant. Brown University, Physics Department, Providence RI (USA) 2006- Summer Undergraduate Research Fellowship. California Institute of Technology, Pasadena, CA (USA) 2006-2007 Undergraduate Teaching Assistant. Syracuse University, Physics Department, Syracuse NY (USA) 2005-2007 Research Assistant. Syracuse University, Physics Department, Syracuse NY (USA) Refereed Publications Corresponding author • D.C. Malling et al., “Rare Event Search Backgrounds from Primordial Radionuclide Chain Disequilibrium.” Submitted to Astropart. Phys. (2013). arXiv:1305.5183 [astro- ph.IM]. iv • D.S. Akerib et al., “An Ultra-Low Background PMT for Liquid Xenon Detectors.” Nucl. Instr. Meth. A703 (2013) 1–6. arXiv:1205.2272 [physics.ins-det]. • D.C. Malling et al., “After LUX: The LZ Program.” Proceedings of the 2011 Meeting of the Division of Particles and Fields of the American Physical Society, eConf C110809 (2011). arXiv:1110.0103 [astro-ph.IM]. Contributing Author • D.S. Akerib et al., “First results from the LUX dark matter experiment at the Sanford Underground Research Facility.” Submitted to Phys. Rev. Lett. (2013). arXiv:1310.8214 [astro-ph.CO]. • D.S. Akerib et al., “The Large Underground Xenon (LUX) Experiment.” Nucl. Instr. Meth. A704 (2013) 111–126. arXiv:1211.3788 [physics.ins-det]. • D.S. Akerib et al., “The LUX Prototype Detector: Heat Exchanger Development.” Nucl. Instr. Meth. A709 (2013) 29–36. arXiv:1207.3665 [physics.ins-det]. • D.S. Akerib et al., “Technical Results from the Surface Run of the LUX Dark Matter Experiment.” Astropart. Phys. 45 (2013) 34–43. arXiv:1210.4569 [astro-ph.IM]. • D.S. Akerib et al., “LUXSim: A Component-Centric Approach to Low-Background Simulations.” Nucl. Instr. Meth. A675 (2012) 63–77. arXiv:1111.2074 [physics.data- an]. Presentations Talks • American Physical Society April Meeting, April 2013, Denver CO • Identification of Dark Matter 2012, Kavli Institue for Cosmological Physics at the University of Chicago, July 2012, Chicago IL • Brookhaven Forum, October 2011, Upton NY v • American Physical Society Division of Particles and Fields, August 2011, Providence RI • American Physical Society April Meeting, February 2010, Washington D.C. Posters • Topics in Astroparticle and Underground Physics 2013, September 2013, Asilomar CA Honors and Awards • 2007 - B.S. Engineering Physics. Magna Cum Laude. • 2006 - Summer Undergraduate Research Fellowship, California Institute of Technol- ogy, Pasadena CA • 2003 - Centennial Scholar, Syracuse University, Syracuse NY vi Acknowledgments This dissertation represents a condensed version of only a few of the fascinating projects that I have had the opportunity to work on since my arrival at Brown in June 2007. I have worked hard, I have been fortunate, and I have been supported in all ways by a number of special people. Here I mention only a selection of those who have had the most direct impact on this dissertation. I have had the good fortune to know and work with many more people throughout my career at Brown, before this at Syracuse, and even earlier. I save their mention for what will undoubtedly be a best-selling autobiography. I thank Rick Gaitskell for an immense number of things. I am deeply grateful for his guidance in matters both LUX and non-LUX, whether in the lab, in the office, or strolling around the east side attempting to find a patch of sunshine in the middle of New England “winter1.” I have been truly inspired by Rick’s ebullient approach to all things. I like to think that, in significant part from his mentoring, I have left Brown after six and a half years not only more knowledgeable, but actually smarter, and with a great deal more confidence. I am thankful to Savvas Koushiappas and Dave Cutts for their extremely helpful guid- ance as my thesis committee, along with Rick. From their input, a great deal of refinement in both accuracy and precision have been made on the work captured in this dissertation. From their questions and comments, along with those from Bob Lanou and Ian Dell’Antonio, a very engaging and entertaining discussion has been had of this work, and of the bigger 1I use quotations because I am from Syracuse; the gloomy, brisk, snowless tantrum thrown by the southern New England area from December through March hardly qualifies as the season that I came to know growing up as “The Culling.” vii picture into which this work fits. I gratefully acknowledge the immense amount of help I received from “Papa” Luiz de Viveiros and Peter “Den Mother” Sorensen during my first years in the group. It has been a pleasure continuing to work with them on LUX after their graduation from Brown, and our intellectual discourse and sessions at the bar were no less engaging or valuable after meeting again in South Dakota. I thank my fellow grads, Jeremy Chapman, Carlos Faham and James Verbus, as well as Simon Fiorucci, Monica Pangilinan, and our annexed Brown- ies, Alastair Currie and Attila Dobi, for their support in everything from our collaborative analysis efforts to begrudging me the walk to Mike’s for lunch two to three times per week. I also owe great thanks to Samuel Chan and Dongqing Huang for enthusiastically taking up the reins as I receded into a cocoon in the days of final analyses and writing. My thanks and appreciation extend out fondly to the entire LUX collaboration, past and present, for creating an incredible community based around an incredible experiment. It has been an honor to be a part of it. I might have long ago drifted offof the planet if not for Alex “GS” Geringer-Sameth, Ryan Michney, Andy Blaeser, Indrit Alex Metaj and Mike Antosh. Thank you all for your camaraderie, and for prodding me to occasionally get physical exercise in these latter months. To GS, an additional great thanks for our late-night brainstorming sessions, which routinely sharpened my thoughts to the point at which solutions would seemingly drop out of thin air (and then would take until daybreak to get written up properly). I also gratefully thank Dina Lloyd for her love and support, and for allowing me to work as long and as hard as I was physically able, but no more than that. I finally wish to thank my parents Glenn and Judith Malling for allowing me to ramble on in my excitement about every detail of these last months. Their love and encouragement has done much more than allow me to complete this dissertation. Their hard work and sacrifices afforded me an environment in which I was encouraged in my passion for science, and then given the opportunity to pursue that passion both in and out of school.
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