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

TL;DR

The MAJORANA DEMONSTRATOR searched for double-beta decay of $^{76}$Ge to excited states of $^{76}$Se, setting new limits on decay half-lives using high purity Germanium detectors with advanced background discrimination.

Contribution

This study provides the first world-leading limits on $^{76}$Ge double-beta decay to excited states using a modular Germanium detector array with high background rejection capabilities.

Findings

Set lower half-life limits for $^{76}$Ge to excited states between 0.56 and 2.1 x 10^{24} years.

Achieved a half-life limit of 0.68 x 10^{24} years for the $2 u$ transition to the first $0^+$ excited state.

Demonstrated the effectiveness of detector granularity in background discrimination.

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 $\beta\beta$-decay with the prompt emission of one or two $\gamma$-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 $(0.56-2.1)\cdot10^{24}$ y. In particular, for the $2\nu$ transition to the first $0^+$ E.S. of $^{76}$Se, a lower half-life limit of $0.68\cdot10^{24}$ at 90% CL was achieved.