Results of the MAJORANA DEMONSTRATOR's Search for Double-Beta Decay of ${}^{76}$Ge to Excited States of ${}^{76}$Se

Abstract

The MAJORANA DEMONSTRATOR is searching for double-beta decay of ${}^{76}$Ge to excited states (E.S.) in ${}^{76}$Se using a modular array of high purity Germanium detectors. ${}^{76}$Ge can decay into three E.S.s of ${}^{76}$Se. The E.S. decays have a clear event signature consisting of a ${\hat{I}}^2{\hat{I}}^2$-decay with the prompt emission of one or two ${\hat{I}}^3$-rays, resulting in with high probability in a multi-site event. The granularity of the DEMONSTRATOR detector array enables powerful discrimination of this event signature from backgrounds. Using 21.3 kg-y of isotopic exposure, the DEMONSTRATOR has set world leading limits for each E.S. decay, with 90% CL lower half-life limits in the range of $(0.56-2.1)\cdot 10^{24}$ 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 $0.68\cdot 10^{24}$ at 90% CL was achieved.