Neutrinoless double beta decay is a process that violates lepton number conservation. It is predicted to occur in extensions of the standard model of particle physics. This Letter reports the results from phase I of the Germanium Detector Array (GERDA) experiment at the Gran Sasso Laboratory (Italy) searching for neutrinoless double beta decay of the isotope ${}^{76}\mathrm{Ge}$. Data considered in the present analysis have been collected between November 2011 and May 2013 with a total exposure of 21.6 kg yr. A blind analysis is performed. The background index is about $1\times {10}^{-2}\mathrm{counts}/(\mathrm{keV}\mathrm{kg}\mathrm{yr})$ after pulse shape discrimination. No signal is observed and a lower limit is derived for the half-life of neutrinoless double beta decay of ${}^{76}\mathrm{Ge}$, $T_{1/2}^{0\nu }>2.1\times {10}^{25}\mathrm{yr}$ (90% C.L.). The combination with the results from the previous experiments with ${}^{76}\mathrm{Ge}$ yields $T_{1/2}^{0\nu }>3.0\times {10}^{25}\mathrm{yr}$ (90% C.L.).

The GERDA experiment searches for the lepton-number-violating neutrinoless double-$\beta$ decay of ${}^{76}\mathrm{Ge}$ (${}^{76}\mathrm{Ge}\to {}^{76}\mathrm{Se}+2e^{-}$) operating bare Ge diodes with an enriched ${}^{76}\mathrm{Ge}$ 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 $1.0_{-0.4}^{+0.6}\times {10}^{-3}\mathrm{counts}/(\mathrm{keV}\mathrm{kg}\mathrm{yr})$ has been achieved; if normalized to the energy resolution this is the lowest ever achieved in any $0\nu \beta \beta$ experiment. No signal is observed and a new 90% C.L. lower limit for the half-life of $8.0\times {10}^{25}\mathrm{yr}$ is placed when combining with our previous data. The expected median sensitivity assuming no signal is $5.8\times {10}^{25}\mathrm{yr}$.

The observation of neutrinoless double-beta decay (0νββ) would show that lepton number is violated, reveal that neu-trinos are Majorana particles, and provide information on neutrino mass. A discovery-capable experiment covering the inverted ordering region, with effective Majorana neutrino masses of 15 − 50 meV, will require a tonne-scale experiment with excellent energy resolution and extremely low backgrounds, at the level of ∼0.1 count /(FWHM·t·yr) in the region of the signal. The current generation ⁷⁶Ge experiments GERDA and the Majorana Demonstrator, utilizing high purity Germanium detectors with an intrinsic energy resolution of 0.12%, have achieved the lowest backgrounds by over an order of magnitude in the 0νββ signal region of all 0νββ experiments. Building on this success, the LEGEND collaboration has been formed to pursue a tonne-scale ⁷⁶Ge experiment. The collaboration aims to develop a phased 0νββ experimental program with discovery potential at a half-life approaching or at 10²⁸ years, using existing resources as appropriate to expedite physics results.

The Gerda collaboration is performing a search for neutrinoless double beta decay of 76Ge with the eponymous detector. The experiment has been installed and commissioned at the Laboratori Nazionali...