Search for neutrinoless double-β decay in Ge76 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-$\ensuremath{\beta}$ decay of $^{76}\mathrm{Ge}$. The Majorana Demonstrator consists of 44.1 kg of Ge detectors (29.7 kg enriched to 88% in $^{76}\mathrm{Ge}$) 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-$\ensuremath{\beta}$ decay in $^{76}\mathrm{Ge}$ with $26.0\ifmmode\pm\else\textpm\fi{}0.5$ kg yr of enriched exposure. With the Demonstrator's energy resolution of 2.53 keV FWHM at ${Q}_{\ensuremath{\beta}\ensuremath{\beta}}$, which is the best among all neutrinoless double-$\ensuremath{\beta}$ 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 $^{76}\mathrm{Ge}$ neutrinoless double-$\ensuremath{\beta}$ decay half-life of $2.7\ifmmode\times\else\texttimes\fi{}{10}^{25}$ yr [90% confidence level (CL)] with a median sensitivity of $4.8\ifmmode\times\else\texttimes\fi{}{10}^{25}$ 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 $11.9\ifmmode\pm\else\textpm\fi{}2.0$ counts/(FWHM t yr).