Abstract. New results for the double beta decay of76 Ge are presented. They are extracted from Data obtained with the Heidelberg-Moscow experiment, which operates five enriched 76 Ge detectors in an extreme low-level environment in the Gran Sasso underground laboratory. The two neutrino accompanied double beta decay is evaluated for the first time for all five detectors with a statistical significance of 47.7 kg y resulting in a half life of T

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)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 × 10(-2) counts/(keV kg 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)Ge, T(1/2)(0ν) >2.1 × 10(25) yr (90% C.L.). The combination with the results from the previous experiments with (76)Ge yields T(1/2)(0ν)>3.0 × 10(25) yr (90% C.L.).

The Heidelberg-Moscow experiment gives the most stringent limit on the Majorana neutrino mass. After 24 kg yr of data with pulse shape measurements, we set a lower limit on the half-life of the 0νββ-decay in 76 Ge of T 0ν 1/2 ≥ 5.7 × 10 25 yr at 90% C.L., thus excluding an effective Majorana neutrino mass greater than 0.2 eV. This allows to set strong constraints on degenerate neutrino mass models.Neutrinoless double beta (0νββ) decay is an extremely sensitive tool to probe theories beyond the standard model (see [1]). While the standard model exactly conserves B-L, 0νββ-decay violates lepton number, and B-L, by two units. The simplest mechanism which can induce 0νββ-decay is the exchange of a Majorana neutrino between the decaying neutrons. Alternatively, any theory that contains lepton number violating interactions can lead to the process. Independently of the underlying mechanism, an observation of the 0νββ-decay would be an evidence for a nonzero Majorana neutrino mass [2]. There are several indications for nonzero neutrino masses, the most stringent ones come from solar and atmospheric neutrino experiments. In particular, the confirmation by Super Kamiokande of the atmospheric neutrino deficit [3], provides strong evidence for neutrino oscillations, although also other solutions are possible [4]. If a neutrino as a hot dark matter (HDM) component is taken into account, then fitting the atmospheric, solar and HDM scales with three neutrinos is only possible in the degenerate mass scenario, where all neutrinos have nearly the same mass, in the order of O(eV) [5]. This would lead to an amplitude for 0νββ-decay mediated by the neutrino mass which is accessible by the present sensitivity of the Heidelberg-Moscow experiment.The Heidelberg-Moscow experiment operates five p-type HPGe detectors in the Gran Sasso Underground Laboratory. The Ge crystals were grown out of 19.2 kg of 86% enriched 76 Ge material. The total active mass of the detectors is 10.96 kg, corresponding to 125.5 mol of 76 Ge, the presently largest source strength of all double beta experiments. Four detectors are placed in a common 30 cm thick lead shielding in a radon free nitrogen atmosphere, surrounded by 10 cm of boron-loaded polyethylene and with two layers of 1 cm thick scintillators on top. The remaining detector is situated in a separate box with 27 cm electrolytical copper and 20 cm lead shielding, flushed with gaseous nitrogen and with 10 cm of boron-loaded polyethylene below the box. A detailed description of the experiment and its background is given in [6]. For a further reduction of the already very low background of the experiment, a pulse shape analysis (PSA) method was developed [7]. The analysis distinguishes between multiple scattered interaction in the Ge crystal, so called multiple site events (MSE) and pointlike interactions, i.e. single site events (SSE). Since double beta decay events belong to the SSE category, the method allows to effectively reduce the background of multiple Compton scattered photons. The probability of...

The GERDA collaboration is performing a search for neutrinoless double beta decay of 76 Ge with the eponymous detector. The experiment has been installed and commissioned at the Laboratori Nazionali del Gran Sasso and has started operation in November 2011. The design, construction and first operational results are described, along with detailed information from the R&D phase.

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