The Search for Double-Beta Decay to Excited States in 76-Ge using the MAJORANA DEMONSTRATOR

Abstract

The discovery of neutrino mass is the first tangible contradiction of the Standard Model of particle physics. Two neutrino double-beta decay (2νββ) is an allowed second order process in the Standard Model that has been observed with half-lives in the range of 10^18 − 10^24 y. Because it involves two neutrino vertices, double-beta decay is a useful tool for studying the properties of neutrinos. In particular, the discovery of neutrinoless double-beta decay (0νββ) would indicate that the neutrino is granted mass by the Majorana mechanism, and provide a means of measuring the mass scale of the neutrino. This would also provide a means for violating Lepton number conservation in the Standard Model, potentially enabling a mechanism for the asymmetric creation of more matter than anti-matter in the universe. 0νββ has never been observed and is the active subject of a variety of experiments, with best half-life limits in the range of 10^25 − 10^26 y. In addition, parent nuclei can double-beta decay into excited states of the daughter nucleus. Observing double-beta decay to excited states (ββ E.S.) is helpful in understanding the nuclear matrix elements that are required for interpretting a 0νββ result. The branching ratios to different daughter nuclear states may also provide sensitivity to additional physics beyond the Standard Model; for example, the 2νββ to 2+ daughter states could indicate violation of the Pauli Exclusion Principle, and measurement of 0νββ to excited states would probe the exchange mechanism underlying 0νββ. The MAJORANA DEMONSTRATOR is measuring double-beta decay in 76 Ge using an array of P-type Point Contact (PPC) High Purity Germanium (HPGe) detectors. The experiment contains 35 detectors totalling 29.8 kg of detector mass that are enriched to 88% in 76-Ge so that the detectors act as both source and detector for ββ-decay; there are an additional 23 detectors totalling 14.4 kg of detector mass with the natural isotopic abundance. The experiment is constructed using ultra-low background materials, in a clean environment located 4850’ underground at the Sanford Underground Research Facility in Lead, SD. Thanks to the granularity of the detector array and the PPC dector geometry, the DEMONSTRATOR is capable of distinguishing single- and multi-site events. The PPC detectors also have the best energy resolution of any current generation experiment, at 2.5 keV in the 2039 keV region of interest for 0νββ. These properties have enabled the experiment to measure one of the lowest background rates of currently running experiments. The experiment is also engaged in searching for ββ E.S.. Excited state events are inherently multi-site due to the prompt emission of a γ-ray; by searching through events that hit multiple detectors, the DEMONSTRATOR is capable of performing a sensitive, low background search for these events. This dissertation will begin by presenting the theoretical motivations for both the searches for 0νββ and ββ-decay to excited states. Next, it will describe the MAJORANA DEMONSTRATOR, with a focus on the elements of the experiment most relevant to the search for ββ E.S.. The DEMONSTRATOR’s simulation framework will be described, along with the simulations necessary to the search for ββ E.S.. The techniques used to perform an optimal search for ββ E.S., and the estimation of the detection efficiency and its uncertainty will then be described. A world-leading result using 22 kg-y of exposure will be presented. Finally, this result will be placed in the context of previous results and current theory, and opportunities to improve on the result will be discussed.