Rare multi-nucleon decays with the full data sets of the Majorana Demonstrator

TL;DR

This paper reports on a comprehensive search for rare multi-nucleon decay modes in germanium isotopes using the full dataset of the Majorana Demonstrator, setting new lifetime limits and improving previous bounds.

Contribution

It provides the first full dataset analysis of multi-nucleon decay modes in $^{76}$Ge, establishing new partial lifetime limits and advancing the search for neutrinoless double-beta decay.

Findings

Set new partial lifetime limits for specific multi-nucleon decay modes.

Improved bounds on the fully inclusive tri-proton decay mode.

Analyzed 64.5 kg yr of enriched exposure with no observed decay signals.

Abstract

The Majorana Demonstrator was an ultra-low-background experiment designed for neutrinoless double-beta decay ($0\nu\beta\beta$) investigation in $^{76}$Ge. Located at the Sanford Underground Research Facility in Lead, South Dakota, the Demonstrator utilized modular high-purity Ge detector arrays within shielded vacuum cryostats, operating deep underground. The arrays, with a capacity of up to 40.4 kg (27.2 kg enriched to $\sim 88\%$ in $^{76}$Ge), have accumulated the full data set, totaling 64.5 kg yr of enriched active exposure and 27.4 kg yr of exposure for natural detectors. Our updated search improves previously explored three-nucleon decay modes in Ge isotopes, setting new partial lifetime limits of $1.83\times10^{26}$ years (90\% confidence level) for $^{76}$Ge($ppp$) $\rightarrow$ $^{73}$Cu e$^+\pi^+\pi^+$ and $^{76}$Ge($ppn$) $\rightarrow$ $^{73}$Zn e$^+\pi^+$. The partial lifetime limit for the fully inclusive tri-proton decay mode of $^{76}$Ge is found to be $2.1\times10^{25}$ yr. Furthermore, we have updated limits for corresponding multi-nucleon decays.