The GERDA (germanium detector array) and its successor the LEGEND (large enriched germanium experiment for neutrinoless ββ decay) experiments search for a hypothetical radioactive decay called “neutrinoless double beta decay” in which two neutrons inside a nucleus decay simultaneously to two protons and two electrons without the emission of neutrinos. This decay is connected to very fundamental questions of particle physics: Are neutrinos their own antiparticles? What is the origin of their masses?
GERDA operates germanium detectors in a 4 m diameter liquid argon bath. The germanium has an enriched 76Ge isotope fraction, one of the few isotopes where neutrinoless double beta decay may be observable. GERDA is currently taking data. It is a leading experiment in this field with the lowest rate of background events. No signal is seen so far and the current sensitivity for a 90% C.L. lower limit on the half lifetime is 1026 years. LEGEND is under construction and will operate a factor of five more detectors and improve the sensitivity by an order of magnitude.