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Measurement of Low-Energy Nuclear-Recoil Quenching Factors In MEASUREMENT OF LOW-ENERGY NUCLEAR-RECOIL QUENCHING FACTORS IN CSI[NA] AND STATISTICAL ANALYSIS OF THE FIRST OBSERVATION OF COHERENT, ELASTIC NEUTRINO-NUCLEUS SCATTERING Grayson Currie Rich 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: P.S. Barbeau J. Engel R. Henning H.J. Karwowski S. Washburn ○c 2017 Grayson Currie Rich ALL RIGHTS RESERVED ii ABSTRACT Grayson Currie Rich: Measurement of low-energy nuclear-recoil quenching factors in CsI[Na] and statistical analysis of the first observation of coherent, elastic neutrino-nucleus scattering (Under the direction of P.S. Barbeau) The COHERENT Collaboration has produced the first-ever observation, with a significance of 6:7σ, of a process consistent with coherent, elastic neutrino-nucleus scattering (CEνNS) as first predicted and described by D.Z. Freedman in 1974 [111]. Physics of the CEνNS process are presented along with its relationship to future measurements in the arenas of nuclear physics, fundamental particle physics, and astroparticle physics, where the newly-observed interaction presents a viable tool for investigations into numerous outstanding questions about the nature of the universe. To enable the CEνNS observation with a 14.6-kg CsI[Na] detector, new measurements of the response of CsI[Na] to low-energy nuclear recoils, which is the only mechanism by which CEνNS is detectable, were carried out at Triangle Universities Nuclear Laboratory; these measurements are detailed and an effective nuclear-recoil quenching factor of 8:78±1:66% is established for CsI[Na] in the recoil-energy range of 5{30 keV, based on new and literature data. Following separate analyses of the CEνNS-search data by groups at the University of Chicago and the Moscow Engineering and Physics Institute, information from simulations, calculations, and ancillary measurements were used to inform statistical analyses of the collected data. Based on input from the Chicago analysis, the number of CEνNS events expected from the Standard Model is 173±48; interpretation as a simple counting experiment finds 136±31 CEνNS counts in the data, while a two-dimensional, profile likelihood fit yields 134±22 CEνNS counts. Details of the simulations, calculations, and supporting measurements are discussed, in addition to the statistical procedures. Finally, potential improvements to the CsI[Na]-based CEνNS measurement are presented along with future possibilities for COHERENT Collaboration, including new CEνNS detectors and measurement of the neutrino-induced neutron spallation process. iii Rabbit faces the neutrino floor iv ACKNOWLEDGEMENTS Significant portions of this thesis were built upon the efforts of many members of The COHERENT Collaboration, but a few individuals are owed specific recognition: Juan Collar The man, the myth, the footnotes, the unspeakable approaches to electrical grounding { Juan's non-material contributions are a bottomless bounty of science and art, kindness and fury. His material contributions, such as providing the CsI[Na] crystal used during the QF measurements presented here, are less poetic but of equal importance. My gratitude: ineffable. Jason Newby Jason's tireless effort at ORNL and the SNS ensured that the COHERENT efforts could proceed so smoothly that some may fail to recognize the integral role he continues to play. Beyond the administrivia, Jason's technical savvy and the accompanying reputation, well earned, far proceeds him: I knew his name and his code years before I would join COHERENT. Beyond the know-how, he is quite simply disarmingly nice. Yuri Efremenko Yuri's indefatiguable advancement of neutrino physics at the SNS laid the groundwork for COHERENT, and he has been a strong advocate for many efforts of importance to the work presented here. Kate Scholberg Seemingly the nucleation site for COHERENT, Kate's leadership through the foggy first stages of collaboration have positioned COHERENT extremely well as it enters a kind of experimental adolescence. Alexey Konovalov and Bjorn Scholz Sorcerers of scintillation, wizards of waveform analysis. Bjorn and Alexey made possible the observation of CEνNS and provide evidence that they still make some physicists like they used to. Recognition and profound appreciation must be given to the technical staff of Triangle Universities Nuclear Laboratory: Jeff Addison, Bret Carlin, John Dunham, Patrick Mulkey, Richard O'Quinn, and Chris Westerfeldt. At various points, the sublime capabilities of the Duke University Instrument Shop and the helpfulness of its staff have been crucial. Bernie Jelnik, Phil Lewis, and Richard Nappi have made possible the realization of numerous experiments, (often) on short notice, and without too loudly complaining about my lack of planning. v The liquid scintillator detectors used in the CsI[Na] QF measurements presented in Chap. 4 were pro- vided by Prof. John Mattingly of the Nuclear Engineering Department at North Carolina State University. Members of the Mattingly research group also provided invaluable assistance during the setup and execution of these measurements: Dr. Jonathan Mueller , Rob Weldon, and Mudit Mishra, were every bit as essential as the detectors they brought over. In the course of analyzing neutron time-of-flight spectra and extracting a neutron beam energy dis- tribution, this work used the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation grant number ACI-1053575. Some additional TOF analysis and the development of the fitting software was carried out using the Duke Computing Cluster (DCC). Tom Milledge of Duke University provided extremely valuable guidance, support, and access to both of the aforementioned computing resources. Jimmy Dorff and Barry Wilson have consistently and deftly aided through the years on a wide range of computing-related issues within the physics department at Duke; their expertise and willingness to offer it reduced the barriers that could be imposed by complicated computing needs. Barry and Jimmy also provided valuable connections to people beyond the physics department whose expertise proved valuable (indeed, it was through these two that I was connected to Tom and the DCC). Rounding out the team of people who let me sudo stupid things and don't yell when they have to clean up my mess are Alex Crowell and Bret Carlin. The community of students, postdocs, and faculty at TUNL enhances the science and the experience of countless late nights. Prof. Diane Markoff is always a steady source of support and assistance. Ron Malone and Forrest Friesen have both contributed expertise, time, patience, deft tandem tuning, and earnest ears for bending. Dr. Jack Silano graciously donated his time to accelerator operation; Dr. Laurie Cumberbatch has an infinite reserve of kind words of support; and conversations with Aaron Swindell are always enriching. Dr. Zane Beckwith (of UNC, having avoided TUNL) is ultimately responsible for the creation of the UNC- Graduate-School-compliant LATEX style file used to typeset this document. The list of TUNL alumni who have indelibly brightened my research experience is long. Stephen Daigle, Johnny Cesaratto, and Paddy Finnerty made nuclear physics look much \cooler" than almost anyone and were amazingly welcoming. Prof. Richard Longland (alumni and back) is always a source of insightful conversation. Mitzi Boswell would always have an interesting idea or two lurking around and helped introduce me to a lot of the physics research lifestyle (she made logging in to check on simulation batch jobs look cool). Jeromy Tompkins and Charles Arnold provided a lot of the experience that would shape my drive to continue at TUNL and delve further into the experimental nuclear/particle physics world; I will never forgive them. Prof. Hugon Karwowski has been a steadfast supporter, advocate, and voice of reason for many years. vi Few are as skilled as Dr. K when it comes to diffusing the tension and frustration that accumulate during a difficult experiment. Prof. Tom Clegg has been a fixture of my time at UNC: he taught the first physics class I took, Fall semester of my freshman year. Embarrassingly, he learned my name, my face, and my inclination towards sleeping through class. Tenzing Joshi, Kareem Kazkaz, and Samuele Sangiorgio of Lawrence Livermore National Lab (at one point or another) helped inform my thinking when it came to redirecting my research. My experience with them, beyond being enjoyable, made a lasting scientific impression and would inspire renewed passion and commitment to the pursuit of interesting physics. My experience at LLNL would lead, ultimately, to my involvement with Phil Barbeau when he began his faculty position at Duke. The most meaningful things don't need embellishment: my time working with Phil has been transformative and significant. It can never be forgotten. The team, and my experience, was tremendously enriched by Long Li, Sam Hedges, Connor Awe, Katrina Miller, Ben Suh, as well as Justin Raybern, making cameo appearances. We come now to the bit where there's no veneer of a professional relationship, and, again, I'm largely without words. These people have stoked the fires of passionate living: Marcus Medley, Scott Brennen, Kelly Clancy, Adrienne Heller, Zach Humphrey. Many people shaped my life for the better. I regret not being able to thank them all. The longer I live apart from her, the more I realize that Nadja Vielot taught me much of what I know about being a socialized adult and what little I know about cooking. She also imprinted upon me an obsession with kitchen cleanliness, which makes me a better person. The Pettigrew family (Charlene, Keith, and Stephen) have been welcoming and supportive through the latter stages of grad school, and they've been understanding of my increasing inattention to social interaction. Erica Pettigrew has been present through the vicissitudes of research. Her support, even when it seems futile, has been profoundly important.
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