GERDA: Final Results and Physics Beyond Neutrinoless Double-Beta Decay

Abstract

The GERDA experiment searched for the lepton number violating neutrinoless double-beta 0υββ decay of 76 Ge. Observation of this decay would provide answers to fundamental problems in particle physics and cosmology, including the origin of neutrino masses and baryon asymmetry in the universe. The GERDA experiment achieved the most stringent lower limit on the half-life of the 0υββ decay of 1.8 • 1026 yr at 90% C.L. (which coincides with the sensitivity) by operating high-purity germanium (HPGe) detectors enriched in 76Ge submerged in liquid argon (LAr). The collaboration could achieve this breakthrough by reducing the background event rate to 5.2 • 10–4 counts/(keV kg yr) at the end-point energy. This unprecedented background index could be achieved by developing unique technologies like utilizing the scintillation light of the LAr to reject efficiently background events that deposit energy simultaneously in the HPGe detectors and in LAr, and the pulse shape discrimination which exploits specific event topologies of backgrounds and signal candidates. Due to the ultra-low background approach the GERDA data is also suited for other rare event searches beyond the 0υββ decay like the search for super-WIMPs.