Search for Neutrinoless Double-β Decay in ^{76}Ge with the Majorana Demonstrator.

Abstract

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 44.1kg of Ge detectors (29.7kg 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 2.5keV FWHM at Q_{ββ} and a very low background with no observed candidate events in 9.95kg 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 240-520meV, 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).