A DIRECT SEARCH FOR DARK MATTER WITH THE MAJORANA DEMONSTRATOR Kristopher Reidar Vorren A dissertation submitted to the faculty at the University of North Carolina at Chapel Hill in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Physics. Chapel Hill 2017 Approved by: Reyco Henning Chris Clemens Jonathan Engel Christian Iliadis John F. Wilkerson c 2017 Kristopher Reidar Vorren ALL RIGHTS RESERVED ii ABSTRACT Kristopher Reidar Vorren: A Direct Search for Dark Matter with the Majorana Demonstrator (Under the direction of Reyco Henning) The Majorana Demonstrator is a neutrinoless double-beta decay experiment cur- rently operating 4850 ft underground in the Sanford Underground Research Facility in Lead, SD. Sub-keV thresholds and excellent low-energy resolution are features of the p-type point- contact high-purity germanium detectors deployed by Majorana, making them ideal for use in direct dark matter searches when combined with Majorana's ultra-low backgrounds. An analysis of data from a 2015 commissioning run of the Demonstrator with 478 kg d of exposure was performed to search for mono-energetic lines in the detectors' energy-spectrum from bosonic dark matter absorption. No dark matter signature was found in the 5-100 keV range, and upper limits were placed on dark bosonic pseudoscalar and vector-electric cou- plings. The same analysis produced null results and upper limits for three additional rare- event searches: Pauli-Exclusion Principle violating decay, solar axions, and electron decay. Improvements made to Majorana since commissioning will result in increased sensitivity to rare-event searches in future analyses. iii ACKNOWLEDGEMENTS The work presented here was only possible because of the support provided by so many people over the last several years. My gratitude is not limited to those specifically acknowl- edged here{there are simply too many people that deserve thanks. I want to first thank everyone on the Majorana collaboration. Despite the great stress that comes along with doing good science, the collaboration stayed focused and achieved significant milestones over the last several years. It has been a great pleasure to work with everyone involved, even when that work took place in a cleanroom, one mile underground. This dissertation would never have been written without the overwhelming support from my collaborators. I am grateful to have been a part of the UNC community and especially the Experimen- tal Nuclear and Astroparticle Physics (ENAP) group. Special thanks goes to my advisor, Reyco Henning, who helped me navigate my way through the unending obstacles that arose throughout graduate school. I have to thank John Wilkerson who always seemed to ask the tough questions that ultimately kept me on course for graduation. The other students and the postdocs that have come and gone also deserve thanks; their support and advice was extremely helpful. My family has my gratitude for their support throughout all my years of education. The unconditional support from my mother on any endeavor has been invaluable. My step-dad, Chester, always impressed upon me the importance of critical thinking. The persistence and dedication of my brother Billy, who followed through on his childhood dream of becoming an airplane pilot, has been huge source of inspiration and motivation to get through school. I will never forget my dad who passed away in 2011: he was one of the most generous people I've ever known and valued education above everything. It's terribly regretful that he's not iv here to read this. Finally, to friends, extended family, all the amazing people I've met traveling for work, and everyone else: thank you. This has been a wild ride, but it's only the beginning of the beginning of the end of the beginning. v TABLE OF CONTENTS LIST OF TABLES .................................... x LIST OF FIGURES ................................... xi LIST OF ABBREVIATIONS AND SYMBOLS ................. xiv 1 Introduction ...................................... 1 1.1 The Case for Dark Matter . 1 1.1.1 Indirect Observational Evidence . 3 1.1.2 Cosmological Models with Dark Matter . 10 1.2 Particle Dark Matter Candidates . 14 1.2.1 Weakly Interacting Massive Particles . 15 1.2.2 Axions . 17 1.2.3 Bosonic keV-Scale Dark Matter . 22 1.3 Outline of Dissertation . 25 2 Semiconductor Detectors and the P-type Point-Contact Germanium De- tector .......................................... 26 2.1 Properties of Semiconductor Detectors . 26 2.1.1 Introduction . 26 2.1.2 The Band Gap Model . 27 2.1.3 Impurities and Dopants . 29 vi 2.1.4 Semiconductor Junction . 30 2.2 High Purity Germanium Detectors . 32 2.2.1 Crystal Fabrication . 33 2.2.2 Conventional Ge Detector Configurations . 35 2.3 PPC Detectors . 36 2.3.1 Noise Characteristics . 36 2.3.2 Detector Resolution . 41 2.3.3 Charge Collection in PPC Detectors . 43 2.3.4 Slow Pulses . 45 2.4 Summary and Discussion . 47 3 The Majorana Demonstrator .......................... 48 3.1 Experiment Overview . 48 3.1.1 Neutrinoless Double-Beta Decay . 48 3.1.2 Experiment Goals . 51 3.2 Experiment Infrastructure and Hardware . 56 3.2.1 The Majorana Lab Infrastructure . 56 3.2.2 Detector, String, and Cryostat Assembly . 59 3.2.3 Detector Shielding . 62 3.3 Module 1 . 63 3.3.1 Configuration . 64 3.3.2 Electronics Readout and Performance . 66 3.3.3 DAQ . 67 vii 3.3.4 Anticipated backgrounds . 69 4 Characterization of Data and Systematics ................... 71 4.1 Data Selection . 71 4.1.1 Data Description . 71 4.1.2 Analysis Live-time and Exposure Determination . 72 4.2 Low-Energy Calibration . 76 4.2.1 MJD Energy-Scale Calibration . 76 4.2.2 Correcting the Low-Energy Calibration . 78 4.2.3 The DS0 Low-Energy Spectrum . 83 4.3 Resolution Measurement . 83 4.3.1 Fitting the Resolution below 100 keV . 84 4.3.2 Resolution Fit Parameters . 86 4.4 Summary of Systematic Parameters . 86 5 Data Cleaning .................................... 88 5.1 Surface Event Removal . 88 5.1.1 The T=E parameter . 88 5.2 Electronic Noise Removal . 92 5.2.1 Tagging and Removing Pulser-Retriggering Events . 92 5.2.2 Transient Pulse Identification and Removal . 95 5.3 Data Cleaning Efficiency . 98 5.3.1 Acceptance Efficiency of the Multiplicity Cut . 98 5.3.2 Acceptance Efficiency of the Pulser-Retriggering Cut . 99 viii 5.3.3 The T=E-Cut Acceptance Efficiency . 100 5.4 Data Cleaning Summary . 104 6 Profile Likelihood Analysis and Results .................... 106 6.1 The Profile Likelihood Method . 106 6.1.1 Constructing the Profile Likelihood Function . 107 6.1.2 Building the Signal and Background Model . 110 6.2 Results . 114 6.2.1 Search for keV-scale bosonic pseudoscalar DM . 115 6.2.2 Vector bosonic dark matter results . 119 6.2.3 Solar axions produced in the 57Fe M1 transition . 120 6.2.4 Pauli Exclusion Violating Decay . 123 6.2.5 Electron Decay . 126 7 Conclusion ....................................... 127 7.1 Overview . 127 7.2 Outlook . 128 BIBLIOGRAPHY .................................... 129 ix LIST OF TABLES 1.1 Parameters from the CMB . 14 3.1 List of select double-beta decaying isotopes . 51 4.1 Detectors' Active Mass . 76 4.2 Cosmogenic Background Isotopes . 79 4.3 Low-Energy 228Th Peaks . 83 4.4 Systematic Parameters . 87 5.1 Data Cleaning Parameters . 105 6.1 Likelihood Analysis Parameters . 115 7.1 Summary of Results . 128 x LIST OF FIGURES 1.1 Doppler shift in NGC 7531 . 6 1.2 Rotation Curve of NGC 6503 . 7 1.3 The Bullet Cluster . 9 1.4 Anisotropy in the CMB . 12 1.5 CMB Power Spectrum . 13 1.6 Predicted WIMP ionization spectrum . 17 1.7 WIMP exclusion plot . 18 1.8 Light shining through wall experiment . 21 1.9 Constraints on vector dark matter . 23 2.1 Band Gap Model in Solids . 28 2.2 Semiconductor Impurities . 30 2.3 Semiconductor Junction Physics . 31 2.4 Germanium zone refinement . 33 2.5 Simplified Czochralski process . 34 2.6 Germanium Detector Configurations . 37 2.7 Noise curve and components . 39 2.8 Detector Drift paths . 44 2.9 PPC Weighting Potential . 45 2.10 PPC Pulse Timing . 46 3.1.