The Majorana Demonstrator's Search for Double-Beta Decay of ${}^{76}$Ge to Excited States of ${}^{76}$Se

Abstract

The Majorana Demonstrator is a neutrinoless double-beta decay search consisting of a low-background modular array of high-purity germanium detectors, $\sim 2/3$ of which are enriched to 88\% in ${}^{76}$Ge. The experiment is also searching for double-beta decay of ${}^{76}$Ge to excited states (e.s.) in ${}^{76}$Se. ${}^{76}$Ge can decay into three daughter states of ${}^{76}$Se, with clear event signatures consisting of a ${\hat{I}}^2{\hat{I}}^2$-decay followed by the prompt emission of one or two ${\hat{I}}^3$-rays. This results with high probability in multi-detector coincidences. The granularity of the Demonstrator detector array enables powerful discrimination of this event signature from backgrounds. Using 41.9~kg-y of isotopic exposure, the Demonstrator has set world leading limits for each e.s.\ decay of ${}^{76}$Ge, with 90\% CL lower half-life limits in the range of $(0.75-4.0)\times 10^{24}$~y. In particular, for the $2\hat{I}½$ transition to the first $0^{+}$ e.s.\ of ${}^{76}$Se, a lower half-life limit of $7.5\times 10^{23}$~y at 90\% CL was achieved.

16 pages, 11 figures