PNNL

Abstract

The Majorana Collaboration is operating an array of high purity Ge detectors to search for neutrinoless double-beta decay in 76Ge. The Majorana Demonstrator is comprised of 44.8 kg (29.7 kg enriched in 76Ge) of Ge detectors in total, 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. Given the unprecedented energy resolution at Q of 2.5 keV FWHM and the very low background with no observed candidate events in 10 kg yr of enriched Ge exposure, we find a lower limit on the half-life of 1:9 x 1025 yr (90% CL). This result constrains the effective Majorana neutrino mass to below 250 to 550 meV, depending on the matrix elements used. In our experimental configuration with the lowest background, the background is 4:0 +3:1(superscript) -2:5 (subscript) counts/(FWHM t yr) at 68% CL.