Results on $\beta\beta$ decay with emission of two neutrinos or Majorons in $^{76}$Ge from GERDA Phase I

TL;DR

This paper reports on a search for Majoron-emitting neutrinoless double beta decay in germanium-76, setting new lower limits on half-lives and providing an improved measurement of the two-neutrino decay half-life.

Contribution

It presents the first limits on Majoron-emitting decay modes in $^{76}$Ge with improved sensitivity and a precise measurement of the two-neutrino decay half-life.

Findings

No evidence of Majoron-emitting decay modes was observed.

Lower half-life limits of about 10^{23} years were established for various spectral indices.

The two-neutrino double beta decay half-life was measured as (1.926 ± 0.095)×10^{21} years.

Abstract

A search for neutrinoless $\beta\beta$ decay processes accompanied with Majoron emission has been performed using data collected during Phase I of the GERmanium Detector Array (GERDA) experiment at the Laboratori Nazionali del Gran Sasso of INFN (Italy). Processes with spectral indices n = 1, 2, 3, 7 were searched for. No signals were found and lower limits of the order of 10$^{23}$ yr on their half-lives were derived, yielding substantially improved results compared to previous experiments with $^{76}$Ge. A new result for the half-life of the neutrino-accompanied $\beta\beta$ decay of $^{76}$Ge with significantly reduced uncertainties is also given, resulting in $T^{2\nu}_{1/2} = (1.926 \pm 0.095)\cdot10^{21}$ yr.