Gerda

GERDA, the GERmanium Detector Array, is a particle-physics experiment at the Gran Sasso National Laboratory in Italy designed to search for neutrinoless double beta decay of the isotope 76Ge. The observation of such decay would indicate that neutrinos are Majorana particles and would have implications for the neutrino mass scale and lepton number conservation.

GERDA uses large, enrichment-grade germanium detectors made from 76Ge, operated as bare diodes submerged in liquid argon within a cryostat. The liquid argon serves as a cooling medium and a shield, while the outer water tank and an active Cherenkov detector reduce external backgrounds. The detectors are calibrated to look for a sharp peak at the decay’s Q-value (about 2039 keV).

Phase I (2011–2013) achieved low background and set a stringent lower limit on the half-life for neutrinoless double beta decay of 76Ge. Phase II (2015–2019) added more detectors and improved background rejection with enhanced pulse-shape analysis and vetoing, achieving a background-free regime in the region of interest. No excess over background was observed; the results set stronger limits on the decay half-life and, correspondingly, on the effective Majorana neutrino mass.

GERDA’s work contributed significantly to the development of the LEGEND project, which aims to scale up germanium-based experiments for greater sensitivity. LEGEND-200 and beyond seek to reach sensitivity to half-lives beyond 10^27 years.

By providing some of the most stringent constraints on neutrinoless double beta decay in 76Ge, GERDA has helped shape contemporary efforts to understand the nature of neutrinos and the violation of lepton number.