Results on β β β β decay with emission of two neutrinos or Majorons in ⁷⁶ 76 Ge from GERDA Phase I

A search for neutrinoless β β 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 β β decay of ⁷⁶ Ge with significantly reduced uncertainties is also given, resulting in T2ν1/2 = (1.926 ± 0.094)× 10²¹ yr.