The Majorana Collaboration is operating an array of high purity Ge detectors to search for neutrinoless double-β decay in ^{76}Ge. The Majorana Demonstrator comprises 44.1 kg of Ge detectors (29.7 kg enriched in ^{76}Ge) split between two modules contained in a low background shield at the Sanford Underground Research Facility in Lead, South Dakota. Here we present results from data taken during construction, commissioning, and the start of full operations. We achieve unprecedented energy resolution of 2.5 keV FWHM at Q_{ββ} and a very low background with no observed candidate events in 9.95 kg yr of enriched Ge exposure, resulting in a lower limit on the half-life of 1.9×10^{25}  yr (90% C.L.). This result constrains the effective Majorana neutrino mass to below 240-520 meV, depending on the matrix elements used. In our experimental configuration with the lowest background, the background is 4.0_{-2.5}^{+3.1}  counts/(FWHM t yr).

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).

The MajoranaDemonstratorwill search for the neutrinoless double-beta(ββ0ν)decay of the isotopeGe with a mixed array of enriched and natural germanium detectors. The observation of this rare decay would indicate that the neutrino is its own antiparticle, demonstrate that lepton number is not conserved, and provide information on the absolute mass scale of the neutrino. The Demonstratoris being assembled at the 4850-foot level of the Sanford Underground Research Facility in Lead, South Dakota. The array will be situated in a low-background environment and surrounded by passive and active shielding. Here we describe the science goals of the Demonstratorand the details of its design.

Abstract. The observation of neutrinoless double-beta decay (0νββ) would show that lepton number is violated, reveal that neutrinos are Majorana particles, and provide information on neutrino mass. A discovery-capable experiment covering the inverted ordering region, with effective Majorana neutrino masses of 15 − 50 meV, will require a tonne-scale experiment with excellent energy resolution and extremely low backgrounds, at the level of ∼0.1 count /(FWHM·t·yr) in the region of the signal. The current generation 76 Ge experiments GERDA and the Majorana Demonstrator, utilizing high purity Germanium detectors with an intrinsic energy resolution of 0.12%, have achieved the lowest backgrounds by over an order of magnitude in the 0νββ signal region of all 0νββ experiments. Building on this success, the LEGEND collaboration has been formed to pursue a tonne-scale 76 Ge experiment. The collaboration aims to develop a phased 0νββ experimental program with discovery potential at a half-life approaching or at 10 28 years, using existing resources as appropriate to expedite physics results.