The GERmanium Detector Array (GERDA) experiment searched for the lepton-number-violating neutrinoless double-$\beta$ decay of $^{76}\mathrm{Ge}$. Observing such a decay would allow to shed light onto the nature of neutrinos and its discovery would have far-reaching implications in cosmology and particle physics.
By operating an array of high purity bare germanium detectors, enriched in $^{76}\mathrm{Ge}$, in an active liquid argon shield aided by pulse shape discrimination of germanium detector signals, GERDA achieved an unprecedentedly low background index of $5.2 \times 10^{-4}$ $\mathrm{counts}/(\mathrm{keV} \cdot \mathrm{kg} \cdot \mathrm{yr})$ in the signal region and was able to surpass the design goal of collecting an exposure of $100$ $\mathrm{kg} \cdot \mathrm{yr}$ in a background-free regime.
With a total exposure of $127.2$ $\mathrm{kg} \cdot \mathrm{yr}$ combining Phase I and Phase II, no signal was observed and a lower limit on the half-life of the neutrinoless double-$\beta$ decay in $^{76}\mathrm{Ge}$ is set at $T_{1/2} > 1.8 \cdot 10^{26}$ $\mathrm{yr}$ at 90% C.L., which coincides with the sensitivity assuming no signal.