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Many extensions of the Standard Model of particle physics explain the dominance of matter over antimatter in our Universe by neutrinos being their own antiparticles. This would imply the existence of neutrinoless double-β decay, which is an extremely rare lepton-number-violating radioactive decay process whose detection requires the utmost background suppression. Among the programmes that aim to detect this decay, the GERDA Collaboration is searching for neutrinoless double-β decay of Ge by operating bare detectors, made of germanium with an enriched Ge fraction, in liquid argon. After having completed Phase I of data taking, we have recently launched Phase II. Here we report that in GERDA Phase II we have achieved a background level of approximately 10 counts keV kg yr . This implies that the experiment is background-free, even when increasing the exposure up to design level. This is achieved by use of an active veto system, superior germanium detector energy resolution and improved background recognition of our new detectors. No signal of neutrinoless double-β decay was found when Phase I and Phase II data were combined, and we deduce a lower-limit half-life of 5.3 × 10 years at the 90 per cent confidence level. Our half-life sensitivity of 4.0 × 10 years is competitive with the best experiments that use a substantially larger isotope mass. The potential of an essentially background-free search for neutrinoless double-β decay will facilitate a larger germanium experiment with sensitivity levels that will bring us closer to clarifying whether neutrinos are their own antiparticles.

Neutrinoless double electron capture is a process that, if detected, would give evidence of lepton number violation and the Majorana nature of neutrinos. A search for neutrinoless double electron capture of [Formula omitted]Ar has been performed with germanium detectors installed in liquid argon using data from Phase I of the GERmanium Detector Array (Gerda) experiment at the Gran Sasso Laboratory of INFN, Italy. No signal was observed and an experimental lower limit on the half-life of the radiative neutrinoless double electron capture of [Formula omitted]Ar was established: [Formula omitted] 3.6 [Formula omitted] 10 [Formula omitted] years at 90% CI.

The detectors are made from germanium with the fraction of the 76Ge isotope enriched from 7.8% to about 87%. Since the source and the detector of Ovßß decay are identical in this calorimetric approach, the detection efficiency is high. A rock overburden of about 3,500 m water equivalent removes the hadronic components of cosmic ray showers and reduces the muon flux at the experiment by six orders of magnitude, to 1.2 muons m-2 h-1.

A search for neutrinoless ... decay processes accompanied with Majoron emission has been performed using data collected during Phase I of the GERmanium Detector Array (GERDA) experiment at the Laboratori Nazionali del Gran Sasso of INFN (Italy). Processes with spectral indices ... were searched for. No signals were found and lower limits of the order of 10... yr on their half-lives were derived, yielding substantially improved results compared to previous experiments with ...Ge. A new result for the half-life of the neutrino-accompanied ... decay of ...Ge with significantly reduced uncertainties is also given, resulting in ... yr.

The results of measurements of natural radioactive isotopes content in different source materials of natural and enriched composition used for CaMoO4 scintillation crystal growing are presented. The crystals are to be used in the experiment to search for double neutrinoless betas-decay of Mo-100.

The GERmanium Detector Array (GERDA) collaboration searched for neutrinoless double-\\(\\beta\\) decay in \\(^{76}\\)Ge with an array of about 40 high-purity isotopically-enriched germanium detectors. The experimental signature of the decay is a monoenergetic signal at Q\\(_{\\beta\\beta}\\) = 2039.061(7)keV in the measured summed energy spectrum of the two emitted electrons. Both the energy reconstruction and resolution of the germanium detectors are crucial to separate a potential signal from various backgrounds, such as neutrino-accompanied double-\\(\\beta\\) decays allowed by the Standard Model. The energy resolution and stability were determined and monitored as a function of time using data from regular \\(^{228}\\)Th calibrations. In this work, we describe the calibration process and associated data analysis of the full GERDA dataset, tailored to preserve the excellent resolution of the individual germanium detectors when combining data over several years.

The GERDA collaboration is performing a sensitive search for neutrinoless double beta decay of \\(^{76}\\)Ge at the INFN Laboratori Nazionali del Gran Sasso, Italy. The upgrade of the GERDA experiment from Phase I to Phase II has been concluded in December 2015. The first Phase II data release shows that the goal to suppress the background by one order of magnitude compared to Phase I has been achieved. GERDA is thus the first experiment that will remain background-free up to its design exposure (100 kg yr). It will reach thereby a half-life sensitivity of more than 10\\(^{26}\\) yr within 3 years of data collection. This paper describes in detail the modifications and improvements of the experimental setup for Phase II and discusses the performance of individual detector components.

Neutrinoless double-\\(\\beta\\) decay of \\(^{76}\\)Ge is searched for with germanium detectors where source and detector of the decay are identical. For the success of future experiments it is important to increase the mass of the detectors. We report here on the characterization and testing of five prototype detectors manufactured in inverted coaxial (IC) geometry from material enriched to 88% in \\(^{76}\\)Ge. IC detectors combine the large mass of the traditional semi-coaxial Ge detectors with the superior resolution and pulse shape discrimination power of point contact detectors which exhibited so far much lower mass. Their performance has been found to be satisfactory both when operated in vacuum cryostat and bare in liquid argon within the GERDA setup. The measured resolutions at the Q-value for double-\\(\\beta\\) decay of \\(^{76}\\)Ge (Q\\(_{\\beta\\beta}\\) = 2039 keV) are about 2.1 keV full width at half maximum in vacuum cryostat. After 18 months of operation within the ultra-low background environment of the GERmanium Detector Array (GERDA) experiment and an accumulated exposure of 8.5 kg\\(\\cdot\\)yr, the background index after analysis cuts is measured to be \\(4.9^{+7.3}_{-3.4}\\times 10^{-4}\\) counts /(keV\\(\\cdot\\)kg\\(\\cdot\\)yr) around Q\\(_{\\beta\\beta}\\). This work confirms the feasibility of IC detectors for the next-generation experiment LEGEND.

The Standard Model of particle physics cannot explain the dominance of matter over anti-matter in our Universe. In many model extensions this is a very natural consequence of neutrinos being their own anti-particles (Majorana particles) which implies that a lepton number violating radioactive decay named neutrinoless double beta (\\(0\\nu\\beta\\beta\\)) decay should exist. The detection of this extremely rare hypothetical process requires utmost suppression of any kind of backgrounds. The GERDA collaboration searches for \\(0\\nu\\beta\\beta\\) decay of \\(^{76}\\)Ge (\\(^{76}\\rm{Ge} \\rightarrow\\,^{76}\\rm{Se} + 2e^-\\)) by operating bare detectors made from germanium with enriched \\(^{76}\\)Ge fraction in liquid argon. Here, we report on first data of GERDA Phase II. A background level of \\(\\approx10^{-3}\\) cts/(keV\\(\\cdot\\)kg\\(\\cdot\\)yr) has been achieved which is the world-best if weighted by the narrow energy-signal region of germanium detectors. Combining Phase I and II data we find no signal and deduce a new lower limit for the half-life of \\(5.3\\cdot10^{25}\\) yr at 90 % C.L. Our sensitivity of \\(4.0\\cdot10^{25}\\) yr is competitive with the one of experiments with significantly larger isotope mass. GERDA is the first \\(0\\nu\\beta\\beta\\) experiment that will be background-free up to its design exposure. This progress relies on a novel active veto system, the superior germanium detector energy resolution and the improved background recognition of our new detectors. The unique discovery potential of an essentially background-free search for \\(0\\nu\\beta\\beta\\) decay motivates a larger germanium experiment with higher sensitivity.

Internal contaminations of \\(^{238}\\)U, \\(^{235}\\)U and \\(^{232}\\)Th in the bulk of high purity germanium detectors are potential backgrounds for experiments searching for neutrinoless double beta decay of \\(^{76}\\)Ge. The data from GERDA Phase~I have been analyzed for alpha events from the decay chain of these contaminations by looking for full decay chains and for time correlations between successive decays in the same detector. No candidate events for a full chain have been found. Upper limits on the activities in the range of a few nBq/kg for \\(^{226}\\)Ra, \\(^{227}\\)Ac and \\(^{228}\\)Th, the long-lived daughter nuclides of \\(^{238}\\)U, \\(^{235}\\)U and \\(^{232}\\)Th, respectively, have been derived. With these upper limits a background index in the energy region of interest from \\(^{226}\\)Ra and \\(^{228}\\)Th contamination is estimated which satisfies the prerequisites of a future ton scale germanium double beta decay experiment.