Results on ${\hat{I}}^2{\hat{I}}^2$ decay with emission of two neutrinos or Majorons in ${}^{76}$Ge from GERDA Phase I

Abstract

A search for neutrinoless ${\hat{I}}^2{\hat{I}}^2$ 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 ${\hat{I}}^2{\hat{I}}^2$ decay of ${}^{76}$Ge with significantly reduced uncertainties is also given, resulting in $T_{1/2}^{2\hat{I}½}=(1.926\pm 0.095)\cdot 10^{21}$ yr.

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