FERMILAB-THESIS-2021-04 Measuring Electron Diffusion and Constraining the Neutral Current π0 Background for Single-Photon Events in MicroBooNE A Dissertation Presented for the Doctor of Philosophy Degree The University of Tennessee, Knoxville Andrew J. Mogan May 2021 © by Andrew J. Mogan, 2021 All Rights Reserved. ii For Mom and Dad, for all their support over the years. iii Acknowledgments Where to even start? The past six years have been a wild ride: living in three different states, moving to work at a national lab, travelling to conferences across the country, a year-long global pandemic, and of course, the culmination of my research into this thesis. I couldn’t have made it this far without the support of innumerable mentors, friends, and family. First, I of course need to thank my advisor, Sowjanya Gollapinni. It was truly a stroke of good fortune that she happened to come to UTK right as I passed my qualifying exam. In addition to the usual advisor duties of guiding my graduate research, she also provided a healthy working environment, which I consider to be invaluable. I remember being told that I wouldn’t be able to work a standard nine-to-five in graduate school, but I’m happy to report this to be false (with only a handful of exceptions). In fact, I firmly believe I’ve made it this far not in spite of working such “short” hours, but because I’ve been allowed to maintain a healthy mental state. For this and all of her advice and support over the years, I can’t thank Sowjanya enough. In addition to my on-paper advisor, in my mind I’ve had two other “co-advisors” during my time at Fermilab: Adam Lister and Mark Ross-Lonergan. These poor, unfortunate souls were shouldered with the unenviable task of mentoring a know-nothing student like me. Thanks to Adam for the continued support on the diffusion analysis, even after moving to a different experiment. We’ll always maintain that the diffusion group is the original DL group. Once I joined the single-photon group, Mark was then forced to share the burden of teaching me such basics as git and statistical fitting methods, both of which still mystify me to this day. Both Adam and Mark showed the patience of a saint as I continuously asked stupid questions and made dumb mistakes, and I thank them for their constant patience and mentorship. iv One of the best parts of working on-site at Fermilab (well, until that whole “global pandemic” thing, anyway) was the access to a swath of experts in the field. Thanks to Michelle Stancari and Bruce Baller for propelling the diffusion analysis toward an actual publication; to Wes Ketchum for explaining makefiles and other computing-related wizardy; to Joseph Zennamo for (loudly) explaining various aspects of our simulation; to Stephen Gardiner for dumbing nuclear physics down to my level; and to Maya Wospakrik, both for explaining our calibration methods and for supporting the diffusion analysis. Believe it or not, there is, in fact, life outside of work, and I need to thank the many friends I’ve made at Fermilab for keeping me sane: Colton, for introducing me to Magic: the Gathering (my wallet, however, does not thank you); Andy and Rhiannon (“Rhiandy,” as they’re properly known), for the wild parties at Site 56 and for explaining the intricacies of British culture to an uncultured American; Davio and Michelle, for subjecting me to such visual nightmares as Zardoz and Dead Alive; Samantha, for seemingly being the only other person who can (almost) match my level of sass; Lauren, for knowing literally everything, always; Afro, for her unique form of what can only be described as “Greek energy”; Ivan, for being one of the few people around here who will actually play Super Smash Bros. with me; Kathryn, solely because I think she would hate getting a sappy shout-out here, and I think that’s funny; Avinay, for near-perfect attendance at the virtual socials (which were huge part of keeping me sane during quarantine); Vinnie B, for being the most “sus” liar I’ve ever seen in Among Us; Ohana, for the delicious Brazilian food and for hosting my post-defense celebration; and of course Gray, for putting up with living 10 feet away from me for three years, and for often being the only one to appreciate my incessant Spongebob references. Perhaps even more shocking than the thought of life outside work is that of life outside of the Fermilab group. Here, I thank all of my non-physicist (or at least, non-neutrino physicist) friends for giving me a much-needed break from these insufferable nerds: Cody, for the longest-running friendship of my life (has it really been fourteen years?) and for carrying me through Monster Hunter; Candace, for her bubbly energy and hilarious color commentary while Cody and I play games; Zac and Mariah, for giving me a much-needed excuse to escape to my hometown during tough times (Etcetera coffeehouse, anyone?) and for all the delicious cooking; Caleb and Hannah, for helping me properly ring in these past v few New Years; Jesse, for the wild late-night adventures in Knoxville (I still feel bad about waking your neighbor up at 5 a.m.); and Michael and Chloe (“Michloe”), for providing me a bed and delicious food when I visit Knoxville, for introducing me to Game Grumps, and for the best wedding experience of my life to date (who could forget the tissue handoff of the century?). And finally, what acknowledgements would be complete without thanking my loving and supportive family? Thanks to Mom and Dad for raising a weird, awkward kid into a slightly-less weird, slightly-less awkward adult, for the financial support through college, and for helping me move all of my furniture (which they bought me) to Illinois; to Rachel, for continuing to play online games with me and for the anime recommendations (I swear I’ll get around to Little Witch Academia someday); and to Rebecca, for all the fancy bourbon and for bringing Jake, the cutest pup ever, to all of our family gatherings. vi Abstract Liquid Argon Time Projection Chambers (LArTPCs) are a rising technology in the field of experimental neutrino physics. LArTPCs use ionization electrons and scintillation light to reconstruct neutrino interactions with exceptional calorimetric and position resolution capa- bilities. Here, I present two analyses conducted in the MicroBooNE LArTPC at Fermilab: a measurement of the longitudinal electron diffusion coefficient, DL, in the MicroBooNE detector and a constraint of the systematic uncertainty on MicroBooNE’s single-photon analysis due to the dominant neutral current (NC) π0 background. Longitudinal electron diffusion modifies the spatial and timing resolution of the detector, and measuring it will help correct for these effects. Furthermore, current measurements of DL in liquid argon are sparse and in tension with one another, making the MicroBooNE measurement especially +0.28 2 valuable. We report a measurement of 3.74−0.29 cm /s. MicroBooNE is searching for single- photon events as a potential explanation for the MiniBooNE low-energy excess (LEE) of electron neutrino-like events, which has been interpreted as evidence for low-mass sterile neutrinos. However, this search is overwhelmed by a large NC π0 background. By performing a sideband selection of NC π0 events, we apply a data-driven rate constraint to the single- photon analysis to reduce the systematic uncertainties. At present, this constraint improves the single-photon analysis’ median sensitivity to the LEE-like signal from 0.9σ to 1.5σ. This sensitivity is expected to improve significantly as more data become available. Both of these measurements will not only benefit MicroBooNE, but also inform future LArTPC experiments. vii Table of Contents 1 Introduction 1 2 Overview of Neutrino Physics 4 2.1 Detecting the Undetectable ........................... 4 2.2 The Standard Model ............................... 5 2.3 Neutrino Oscillations ............................... 7 2.4 Neutrino Interactions ............................... 10 2.4.1 Quasielastic Interactions ......................... 12 2.4.2 Resonant Interactions ........................... 12 2.4.3 Other Interactions ............................ 14 2.4.4 Summary ................................. 15 2.5 Open Questions in Neutrino Physics ...................... 16 3 Neutrino Experiments 19 3.1 Solar Neutrino Experiments ........................... 20 3.2 Reactor Neutrino Experiments .......................... 21 3.3 Atmospheric Neutrino Experiments ....................... 21 3.4 Accelerator-Based Neutrino Experiments .................... 22 3.5 Short-Baseline Accelerator Anomalies ...................... 24 4 The MicroBooNE Experiment 30 4.1 Booster Neutrino Beam at Fermilab ....................... 31 4.2 The MicroBooNE Detector ............................ 35 viii 4.3 Data Acquisition Readout Electronics ...................... 38 4.4 Ionization Signal ................................. 40 4.5 Light Collection .................................. 42 4.6 UV Laser System ................................. 44 4.7 Cosmic Ray Tagger System ........................... 44 4.8 Data-Taking Triggers ............................... 47 4.9 Detector Operations ............................... 48 5 Simulation and Reconstruction in MicroBooNE 51 5.1 Particle Generation and Propagation ...................... 51 5.2 Signal Processing ................................. 53 5.3 Reconstruction of Particle Objects ........................ 56 5.3.1 Optical Signal Reconstruction ...................... 56 5.3.2 Ionization Signal Reconstruction ..................... 57 5.4 Calorimetry