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

A search for neutrinoless $ββ$ 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 $ββ$ decay of $^{76}$Ge with significantly reduced uncertainties is also given, resulting in $T^{2ν}_{1/2} = (1.926 \pm 0.095)\cdot10^{21}$ yr.