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Abstract

The search for neutrinoless double-beta decay (0νββ) is one of the most active fields in modern particle physics as the observation of this process would prove lepton number violation and imply new physics beyond the Standard Model of particle physics. The Gerda experiment searches for this decay by operating bare Germanium detectors, enriched in the ββ isotope ⁷⁶Ge, in liquid argon. For the first time, a ββ-experiment combines the excellent properties of semiconductor Germanium detectors with an active background suppression technique based on the simultaneous detection of liquid argon scintillation light by photomultiplier tubes and silicon photomultipliers coupled to scintillating fibers (LAr veto). The LAr veto has been successfully operated during the first six months of Phase II of the experiment and yielded - in combination with a Germanium detector pulse shape discrimination technique - a background index of (0.7^+1:1_-0.5) 10^-3 (^cts/_{kg· keV·yr} ). With an ultimate exposure of 100 kg ·yr this will allow for a 0νββ-decay half-life sensitivity of the Gerda Phase II experiment of 10²⁶ yr. Double-beta decay under the emission of two neutrinos (2 νββ) is a second-order process but which is allowed by the Standard Model. The excellent background reduction of the LAr veto results in an unprecedented signal-to-background ratio of 30:1 in the energy region dominated by 2 νββ-decay of ⁷⁶Ge. The remaining background after LAr veto is estimated using the suppression factor from calibration source measurements and results in a measurement of T^2ν_1/2 = (1.98 ± 0.02 (stat) ± 0.05 (syst)) · 10²¹ yr and of T^2ν_1/2 = (1.92 ± 0.02 (stat) ± 0.11 (syst)) · 10²¹ yr based on two different detector designs and given uncertainties on the detector parameters but both with improved systematic uncertainties in comparison to earlier measurements.