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

TL;DR

The Majorana Demonstrator searches for neutrinoless double-beta decay of $^{76}$Ge to excited states of $^{76}$Se, setting world-leading limits on the half-lives of these rare processes using a low-background germanium detector array.

Contribution

This work provides the first limits on double-beta decay to excited states of $^{76}$Se using the Majorana Demonstrator, demonstrating the detector's capability to probe rare decay modes.

Findings

Set lower half-life limits for $^{76}$Ge decay to excited states, up to $(0.75-4.0) imes 10^{24}$ years.

Achieved a half-life limit of $7.5 imes 10^{23}$ years for the $2 u$ transition to the first $0^+$ excited state.

Demonstrated the detector array's effectiveness in background discrimination for rare decay searches.

Abstract

The Majorana Demonstrator is a neutrinoless double-beta decay search consisting of a low-background modular array of high-purity germanium detectors, $\sim2/3$ 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 $\beta\beta$-decay followed by the prompt emission of one or two $\gamma$-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 $(0.75-4.0)\times10^{24}$~y. In particular, for the $2\nu$ transition to the first $0^+$ e.s.\ of $^{76}$Se, a lower half-life limit of $7.5\times10^{23}$~y at 90\% CL was achieved.