Results on Neutrinoless Double- β Decay of Ge76 from 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 $1ifmmodetimeselsetexttimesfi{}{10}^{ensuremath{-}2}text{ }text{ }mathrm{counts}/(mathrm{keV}text{ }mathrm{kg}text{ }mathrm{yr})$ after pulse shape discrimination. No signal is observed and a lower limit is derived for the half-life of neutrinoless double beta decay of $^{76}mathrm{Ge}$, ${T}_{1/2}^{0ensuremath{nu}}>2.1ifmmodetimeselsetexttimesfi{}{10}^{25}text{ }text{ }mathrm{yr}$ (90% C.L.). The combination with the results from the previous experiments with $^{76}mathrm{Ge}$ yields ${T}_{1/2}^{0ensuremath{nu}}>3.0ifmmodetimeselsetexttimesfi{}{10}^{25}text{ }text{ }mathrm{yr}$ (90% C.L.).