Results on Neutrinoless Double-βDecay ofGe76from Phase I of the GERDA Experiment

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 $1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}\text{ }\text{ }\mathrm{counts}/(\mathrm{keV}\text{ }\mathrm{kg}\text{ }\mathrm{yr})$ after pulse shape discrimination. No signal is observed and a lower limit is derived for the half-life of neutrinoless double beta decay of $^{76}\mathrm{Ge}$, ${T}_{1/2}^{0\ensuremath{\nu}}g2.1\ifmmode\times\else\texttimes\fi{}{10}^{25}\text{ }\text{ }\mathrm{yr}$ (90% C.L.). The combination with the results from the previous experiments with $^{76}\mathrm{Ge}$ yields ${T}_{1/2}^{0\ensuremath{\nu}}g3.0\ifmmode\times\else\texttimes\fi{}{10}^{25}\text{ }\text{ }\mathrm{yr}$ (90% C.L.).