A Search for Neutrinoless Double-Beta Decay in 76 Ge 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-beta 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 ultra-clean 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-beta decay in 76 Ge with 26 . 0 ± 0 . 5 kg-yr of enriched exposure. With the Demonstrator ’s unprecedented energy resolution of 2.53 keV FWHM at Q ββ , 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-beta decay half-life of 2 . 7 × 10 25 yr (90% 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 low-background configurations is 11 . 9 ± 2 . 0 counts/(FWHM t yr).