F. Salamida

The GERmanium Detector Array (GERDA) experiment searched for the lepton-number-violating neutrinoless double-β (0νββ) decay of ^{76}Ge, whose discovery would have far-reaching implications in cosmology and particle physics. By operating bare germanium diodes, enriched in ^{76}Ge, in an active liquid argon shield, GERDA achieved an unprecedently low background index of 5.2×10^{-4} counts/(keV kg yr) in the signal region and met the design goal to collect an exposure of 100 kg yr in a background-free regime. When combined with the result of Phase I, no signal is observed after 127.2 kg yr of total exposure. A limit on the half-life of 0νββ decay in ^{76}Ge is set at T_{1/2}>1.8×10^{26} yr at 90% C.L., which coincides with the sensitivity assuming no signal.

The GERDA experiment searches for the lepton-number-violating neutrinoless double-β decay of ^{76}Ge (^{76}Ge→^{76}Se+2e^{-}) operating bare Ge diodes with an enriched ^{76}Ge fraction in liquid argon. The exposure for broad-energy germanium type (BEGe) detectors is increased threefold with respect to our previous data release. The BEGe detectors feature an excellent background suppression from the analysis of the time profile of the detector signals. In the analysis window a background level of 1.0_{-0.4}^{+0.6}×10^{-3} counts/(keV kg yr) has been achieved; if normalized to the energy resolution this is the lowest ever achieved in any 0νββ experiment. No signal is observed and a new 90% C.L. lower limit for the half-life of 8.0×10^{25} yr is placed when combining with our previous data. The expected median sensitivity assuming no signal is 5.8×10^{25} yr.

Looking for an exotic decay Neutrinos—elementary fermionic particles with no electrical charge—defy the standard model of particle physics by having a tiny, but nonzero mass. One explanation for their properties is that they are Majorana fermions, which are particles equal to their antiparticles. If neutrinos were Majorana fermions, a process called neutrinoless double-β decay would become possible: an unstable nucleus could decay by turning two of its neutrons into protons with the emission of two electrons but no antineutrinos. The GERDA Collaboration searched for this decay in a particular isotope of germanium. Housed deep underground to reduce the background signal, the experiment did not detect the elusive process but did place improved boundaries on its half-life. Science , this issue p. 1445

The Gerda collaboration is performing a sensitive search for neutrinoless double beta decay of $$^{76}\hbox {Ge}$$ at the INFN Laboratori Nazionali del Gran Sasso, Italy. The upgrade of the Gerda experiment from Phase I to Phase II has been concluded in December 2015. The first Phase II data release shows that the goal to suppress the background by one order of magnitude compared to Phase I has been achieved. Gerda is thus the first experiment that will remain “background-free” up to its design exposure ( $$\hbox {100 kg}~\hbox {year}$$ ). It will reach thereby a half-life sensitivity of more than $$10^{26}$$ year within 3 years of data collection. This paper describes in detail the modifications and improvements of the experimental setup for Phase II and discusses the performance of individual detector components.