The GERDA experiment searches for neutrinoless double beta (0νββ) decay of 76Ge using high purity germanium (HPGe) detectors operated in liquid argon (LAr). GERDA relies on improved active background reduction techniques such as pulse shape discrimination (PSD). Phase II of the experiment includes a major upgrade: for further background rejection, the LAr cryostat is instrumented to detect argon scintillation light (LAr veto).
With an energy resolution of better than 0.2% (FWHM) in the region of interest (ROI) and a background index (BI) of the order of 10-3 counts/keV/kg/yr, GERDA Phase II is a high resolution, background free experiment. With this unprecedented BI, less than one background event is expected until an exposure of 100 kg yr. This is the lowest background in the ROI achieved so far with respect to other isotopes.
With a collected exposure of 21 kg yr in Phase I and 23 kg yr in Phase II, the GERDA experiment sets a new limit on the half-life of 0νββ decay of 76Ge of T1/20ν > 8.0 · 1025 yr at 90% C.L., with a median sensitivity of T1/20ν > 5.8 · 1025 yr at 90% C.L.