An Improved Background Model and Two-Neutrino Double-Beta Decay Measurement for the MAJORANA DEMONSTRATOR

Neutrinoless double-beta decay (0νββ) is a hypothesized nuclear process that would provide direct evidence of physics beyond the Standard Model in the form of lepton number violation. Its detection would prove that neutrinos are their own antiparticles and play a role in addressing questions of how neutrinos acquire their mass and why the universe exhibits a matter-antimatter asymmetry. Since 0νββ would have an extremely long half-life, one of the key challenges for any 0νββ experiment is mitigating radioactive backgrounds that could obscure a signal.

The MAJORANA DEMONSTRATOR was a germanium-based 0νββ decay search that operated at the Sanford Underground Research Facility and set a 90% CL lower limit of 8.3 x 10²⁵ yr on the 0νββ half-life in ⁷⁶Ge. Through the use of a multi-layer passive shield, radiopure materials, and analysis-based background rejection techniques, the DEMONSTRATOR achieved a background index of 6.23 ^+0.55_-0.52 x 10^-3 c/(keV kg yr) in its low-background configuration. This is one of the lowest achieved backgrounds in a 0νββ experiment, but it is in excess of the assay-based projection by over a factor of five.

Frequentist fits to the DEMONSTRATOR’s data allow its energy spectrum to be decomposed into contributions from different decay chains and experimental components. This talk details the development and results of the DEMONSTRATOR’s data-driven background model, including its conclusions about the source of the background excess. The resulting model also enabled a precision measurement of the half-life of 0νββ’s Standard Model counterpart, two-neutrino double-beta decay, one of the rarest processes ever detected. In this presentation, I will describe the results of this measurement and the systematic studies performed to quantify the half-life uncertainty. I will also discuss how this work has informed the design of the next-generation ton-scale experiment, LEGEND-1000, and its precursor, LEGEND-200. LEGEND-1000 aims to achieve sensitivities to 0νββ half-lives beyond 10²⁸ years, covering the entire inverted ordering parameter space.

Anna Reine- UNC