UNC-CH Physics and Astronomy Thesis Proposal Presentation

Christopher Haufe

“A Comprehensive Background Model for the Majorana Demonstrator”

The Majorana Demonstrator is an experiment constructed to search for neutrinoless double beta decays in 76Ge and to demonstrate the feasibility to deploy a tonne-scale experiment in a phased and modular fashion. It consists of two modular arrays of natural and 76Ge-enriched germanium p-type point contact detectors totaling 44.1 kg, located at the 4850′ level of the Sanford Underground Research Facility in Lead, South Dakota, USA. The DEMONSTRATOR has been operating since June 2015 and has produced six datasets to achieve a neutrinoless double beta decay half-life lower limit of > 2.7 x 10^25 yr (90% C.L.). The latest release of data from the DEMONSTRATOR revealed a background in the region of interest that is higher than projected: 11.9 plus/minus 2.0 counts/(FWHM-tonne-yr) from active exposure. This is more than five times greater than the projected background count of 2.2 counts/(FWHM-tonne-yr) based on assay data. This result emphasizes the importance of understanding and quantifying the sources of backgrounds in the DEMONSTRATOR, especially as similar background mitigation techniques will be used for the tonne-scale 76Ge-based LEGEND experiment. A rigorous background model that builds on previous efforts is under development to identify these sources and more accurately quantify the background energy spectrum.