Search for neutrinoless double-β decay in 76Ge with 26 kg yr of exposure from the Muc(ajorana) Duc(emonstrator)

The Muc(ajorana) Collaboration is operating an array of high-purity Ge detectors to search for the neutrinoless double-β decay of ⁷⁶Ge. The Muc(ajorana) Duc(emonstrator) 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 Duc(emonstrator)'s energy resolution of 2.53 keV FWHM at Q_{β 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).