Search for Neutrinoless Double Beta Decay Using the Full Majorana Demonstrator Dataset

Abstract

The \textsc{Majorana} Collaboration is searching for the neutrinoless double-$\beta$ decay of $^{76}$Ge. The \textsc{Majorana Demonstrator} consists of two modular arrays of high-purity Ge detectors operated in vacuum cryostats housed in a low-background shield at the Sanford Underground Research Facility in Lead, South Dakota. The arrays have operated with up to 48.5~kg of detectors (30.9~kg enriched to $\sim$88\% in $^{76}$Ge). The \textsc{Demonstrator} has completed operation of its enriched detectors, and a limit has been produced with $64.5\pm0.9$~kg-yr of enriched exposure. This thesis describes the development and implementation of the DCR (Delayed Charge Recovery) estimator, used for surface alpha background rejection. The properties of the alpha spectrum described herein show the absolutely critical importance of surface alpha background rejection, and DCR in particular, in the performance of the \textsc{Majorana Demonstrator}. The thesis also describes a parallel, independent statistical analysis of the full \textsc{Majorana Demonstrator} $^{76}$Ge dataset, yielding a $0\nu\beta\beta$ decay half-life lower limit of $8.4 \times 10^{25}$ years, and an upper limit on $m_{\beta\beta}$ of $112-267$~meV, depending on the nuclear matrix element used.