Assay-based background projection for the Majorana Demonstrator using Monte Carlo uncertainty propagation

This is the authors' abstract. We don't add key points for this paper.

The background index (BI) is an important quantity to project and calculate the half-life sensitivity of neutrinoless double-β decay (0νββ) experiments. An analysis framework is presented to calculate the BI using the specific activities, masses, and simulated efficiencies of an experiments components as distributions. This Bayesian framework includes a unified approach to combine specific activities from assay. Monte Carlo uncertainty propagation is used to build a BI distribution from the specific activity, mass, and efficiency distributions. This method is applied to the Majorana Demonstrator, which deployed arrays of high-purity Ge detectors enriched in ⁷⁶Ge to search for 0νββ. The original assay-based projection is requantified in the new framework, using the as-built geometry of the Demonstrator and additional assay information. While 47% higher than the original projection, the resulting BI of $[8.95±{}0.36]×{}{10}^{{-}4}{{3.33333pt}{0ex}}cts/(keV{{0.16em}{0ex}}kg{{0.16em}{0ex}}yr)$ from the $²³²Th$ and $²³⁸U$ decay chains does not account for the higher-than-expected BI observed by the Demonstrator. This method enables us to demonstrate the statistical incompatibility between the Demonstrator's observed background and the assay results.