Search for tri-nucleon decays of $^{76}$Ge in GERDA

TL;DR

This study searches for rare tri-nucleon decay processes in germanium-76 using GERDA data, setting new lower limits on their lifetimes and significantly improving previous constraints.

Contribution

It provides the first comprehensive search for all tri-nucleon decay modes in $^{76}$Ge and establishes the most stringent lifetime limits to date.

Findings

No signal candidates found for tri-nucleon decays.

Lower lifetime limit of 1.2x10^{26} years established.

Improves previous decay lifetime limits by one to three orders of magnitude.

Abstract

We search for tri-nucleon decays of $^{76}$Ge in the dataset from the GERmanium Detector Array (GERDA) experiment. Decays that populate excited levels of the daughter nucleus above the threshold for particle emission lead to disintegration and are not considered. The ppp-, ppn-, and pnn-decays lead to $^{73}$Cu, $^{73}$Zn, and $^{73}$Ga nuclei, respectively. These nuclei are unstable and eventually proceed by the beta decay of $^{73}$Ga to $^{73}$Ge (stable). We search for the $^{73}$Ga decay exploiting the fact that it dominantly populates the 66.7 keV $^{73m}$Ga state with half-life of 0.5 s. The nnn-decays of $^{76}$Ge that proceed via $^{73m}$Ge are also included in our analysis. We find no signal candidate and place a limit on the sum of the decay widths of the inclusive tri-nucleon decays that corresponds to a lower lifetime limit of 1.2x10$^{26}$ yr (90% credible interval). This result improves previous limits for tri-nucleon decays by one to three orders of magnitude.