A Search for Neutrinoless Double-Beta Decay in 76Ge with 26 kg-yr of Exposure from the MAJORANA DEMONSTRATOR

The Majorana Collaboration is operating an array of high-purity Ge detectors to search for the neutrinoless double-𝛽 decay of ⁷⁶Ge. The Majorana Demonstrator consists of 44.1 kg of Ge detectors (29.7 kg enriched to 88% in ⁷⁶Ge) split between two modules constructed from ultraclean materials. Both modules are contained in a low-background shield at the Sanford Underground Research Facility in Lead, South Dakota. We present updated results on the search for neutrinoless double-𝛽 decay in ⁷⁶Ge with 26.0±0.5 kg yr of enriched exposure. With the Demonstrator's energy resolution of 2.53 keV FWHM at 𝑄𝛽⁢𝛽, which is the best among all neutrinoless double-𝛽 decay experiments, we observe one event in the region of interest with 0.65 events expected from the estimated background, resulting in a lower limit on the 76Ge neutrinoless double-𝛽 decay half-life of 2.7×10²⁵ yr [90% confidence level (CL)] with a median sensitivity of 4.8×10²⁵ yr (90% CL). Depending on the matrix elements used, a 90% CL upper limit on the effective Majorana neutrino mass in the range of 200–433 meV is obtained. The measured background in the configurations with full shielding and optimized grounding is 11.9±2.0 counts/(FWHM t yr).