Results on ββ decay with emission of two neutrinos or Majorons in ⁷⁶Ge 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 10²³ yr on their half-lives were derived, yielding substantially improved results compared to previous experiments with ⁷⁶Ge. A new result for the half-life of the neutrino-accompanied ββ decay of ⁷⁶Ge with significantly reduced uncertainties is also given, resulting in T_1/2^2ν=(1.926±0.094)×10²¹ yr