Search for neutrinoless double-β decay in ^{76}Ge with 26 kg yr of exposure from the Majorana Demonstrator

Abstract

The Majorana Collaboration is operating an array of high-purity Ge detectors to search for the neutrinoless double-β decay of ^{76}Ge. The Majorana Demonstrator consists of 44.1 kg of Ge detectors (29.7 kg enriched to 88% in ^{76}Ge) 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 ^{76}Ge with 26.0±0.5 kg yr of enriched exposure. With the Demonstrator's energy resolution of 2.53 keV FWHM at Q_{ββ}, 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 ^{76}Ge neutrinoless double-β decay half-life of 2.7×10^{25} yr [90% confidence level (CL)] with a median sensitivity of 4.8×10^{25} 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).