Agostini M. et al. (GERDA Collaboration) M. Wójcik, G. Zuzel, K. Pelczar, K. Panas, M. Misiaszek, N. Frodyma, D. Borowicz

Download

http://dx.doi.org/10.1140/epjc/s10052-015-3627-y

Abstract

A search for neutrinoless ββ 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 n=1,2,3,7 were searched for. No signals were found and lower limits of the order of 1023 yr on their half-lives were derived, yielding substantially improved results compared to previous experiments with 76Ge. A new result for the half-life of the neutrino-accompanied ββ decay of 76Ge with significantly reduced uncertainties is also given, resulting in T2ν1/2=(1.926±0.094)×1021 yr.

Agostini M. et al. (GERDA Collaboration) M. Wójcik, G. Zuzel, K. Pelczar, K. Panas, M. Misiaszek, N. Frodyma, D. Borowicz

Download

http://iopscience.iop.org/article/10.1088/0954-3899/42/11/115201

Abstract
Two neutrino double beta decay of 76-Ge to excited states of 76-Se has been studied using data from Phase I of the GERDA experiment. An array composed of up to 14 germanium detectors including detectors that have been isotopically enriched in 76-Ge was deployed in liquid argon. The analysis of various possible transitions to excited final states is based on coincidence events between pairs of detectors where a de-excitation γ ray is detected in one detector and the two electrons in the other. No signal has been observed and an event counting profile likelihood analysis has been used to determine Frequentist 90% C.L. bounds for three transitions:

Agostini M. et al. (GERDA Collaboration) M. Wójcik, G. Zuzel, K. Pelczar, K. Panas, M. Misiaszek, N. Frodyma, D. Borowicz

Download

http://dx.doi.org/10.1140/epjc/s10052-015-3409-6

Abstract

An optimized digital shaping filter has been developed for the Gerda experiment which searches for neutrinoless double beta decay in 76Ge. The Gerda Phase I energy calibration data have been reprocessed and an average improvement of 0.3 keV in energy resolution (FWHM) corresponding to 10 % at the Q value for 0νββ decay in 76Ge is obtained. This is possible thanks to the enhanced low-frequency noise rejection of this Zero Area Cusp (ZAC) signal shaping filter.

Agostini M., Barnabé-Heider M., Budjáš D., Cattadori C., Gangapshev A., Gusev K., Heisel M., Junker M., Klimenko A., Lubashevskiy A., Pelczar K. , Schönert S., Smolnikov A., Zuzel G.

Download

http://dx.doi.org/10.1140/epjc/s10052-015-3681-5

Abstract

LArGe is a Gerda low-background test facility to study novel background suppression methods in a low-background environment, for future application in the Gerda experiment. Similar to Gerda, LArGe operates bare germanium detectors submersed into liquid argon (1 m^{3}, 1.4 tons), which in addition is instrumented with photomultipliers to detect argon scintillation light. The scintillation signals are used in anti-coincidence with the germanium detectors to effectively suppress background events that deposit energy in the liquid argon. The background suppression efficiency was studied in combination with a pulse shape discrimination (PSD) technique using a BEGe detector for various sources, which represent characteristic backgrounds to Gerda. Suppression factors of a few times 10^{3} have been achieved. First background data of LArGe with a coaxial HPGe detector (without PSD) yield a background index of (0.12−4.6) \times 10^{−2} cts/(keV kg year) (90 % C.L.), which is at the level of Gerda Phase I. Furthermore, for the first time we monitor the natural ^{42}Ar abundance (parallel to Gerda), and have indication for the 2\nu \beta \beta-decay in natural germanium. These results show the effectivity of an active liquid argon veto in an ultra-low background environment. As a consequence, the implementation of a liquid argon veto in Gerda Phase II is pursued.

Agostini M. et al. (GERDA Collaboration) M. Wójcik, G. Zuzel, K. Pelczar, M. Misiaszek, N. Frodyma, D. Borowicz

Download

http://dx.doi.org/10.1140/epjc/s10052-014-3253-0

Abstract

The GERmanium Detector Array (Gerda) at the Gran Sasso Underground Laboratory (LNGS) searches for the neutrinoless double beta decay (0νββ) of 76Ge. Germanium detectors made of material with an enriched 76Ge fraction act simultaneously as sources and detectors for this decay. During Phase I of theexperiment mainly refurbished semi-coaxial Ge detectors from former experiments were used. For the upcoming Phase II, 30 new 76Ge enriched detectors of broad energy germanium (BEGe)-type were produced. A subgroup of these detectors has already been deployed in Gerda during Phase I. The present paper reviews the complete production chain of these BEGe detectors including isotopic enrichment, purification, crystal growth and diode production. The efforts in optimizing the mass yield and in minimizing the exposure of the 76Ge enriched germanium to cosmic radiation during processing are described. Furthermore, characterization measurements in vacuum cryostats of the first subgroup of seven BEGe detectors and their long-term behavior in liquid argon are discussed. The detector performance fulfills the requirements needed for the physics goals of Gerda Phase II.

Results on Neutrinoless Double-β Decay of ⁷⁶Ge from Phase I of the GERDA Experiment

The neutrino could be its own antiparticle, and this dual role could cause neutrinos to disappear from certain nuclear decays. Tentative evidence for neutrinoless decays appeared in 2004, in conflict with other experimental searches. A new experiment called Gerda has now conclusively refuted this positive result. The nondetection, reported in Physical Review Letters, places the most stringent limits yet on how frequently neutrinoless decays could occur.

Saying that neutrinos and antineutrinos have the same identity (as described by Ettore Majorana in 1932) conflicts with the standard model of particle physics, but it might explain why neutrinos have a much smaller mass than other particles. One way to test for the neutrino’s true nature is with double beta decay, in which two neutrons decay simultaneously into a pair of protons, electrons, and antineutrinos. In the Majorana picture, the same reaction could occur with an antineutrino emitted by one neutron being absorbed as a neutrino by a second neutron. In this neutrinoless double beta decay, the electrons would carry away all the emitted energy.

The Germanium Detector Array (or Gerda), located in the INFN Gran Sasso National Laboratory in Italy, reproduces the earlier 2004 experiment but with improved background suppression. The researchers indirectly detect neutrinos from the double beta decay of germanium-76 by measuring the energy spectrum of the emitted electrons. Neutrinoless decays would cause a peak in the number of events at the maximum electron energy. The Gerda collaboration (Agostini et al.) found no such peak in their data, which implies the rate of neutrinoless double beta decay—if it occurs at all—is less than 10−4 that of normal double beta decay. – Michael Schirber

M Agostini et al., (GERDA Collaboration), N. Frodyma, M. Misiaszek, K. Pelczar, M. Wojcik, G. Zuzel

Download

http://arxiv.org/abs/1306.5084
http://link.aps.org/doi/10.1103/PhysRevLett.111.122503

Abstract

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 76Ge. Data considered in the present analysis have been collected between November 2011 and May 2013 with a total exposure of 21.6 kgyr. A blind analysis is performed. The background index is about 1.10^{-2} cts/(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 76Ge, T_1/2 > 2.1 10^{25} yr (90% C.L.). The combination with the results from the previous experiments with 76Ge yields T_1/2 > 3.0 10^{25} yr (90% C.L.).

M Agostini et al., (GERDA Collaboration), N. Frodyma, M. Misiaszek, K. Pelczar, M. Wojcik, G. Zuzel

Download

http://arxiv.org/abs/1306.5084

http://link.springer.com/article/10.1140/epjc/s10052-013-2583-7

Abstract

The GERDA experiment located at the LNGS searches for neutrinoless double beta (0\nu\beta\beta) decay of ^{76}Ge using germanium diodes as source and detector. In Phase I of the experiment eight semi-coaxial and five BEGe type detectors have been deployed. The latter type is used in this field of research for the first time. All detectors are made from material with enriched ^{76}Ge fraction. The experimental sensitivity can be improved by analyzing the pulse shape of the detector signals with the aim to reject background events. This paper documents the algorithms developed before the data of Phase I were unblinded. The double escape peak (DEP) and Compton edge events of 2.615 MeV \gamma\ rays from ^{208}Tl decays as well as 2\nu\beta\beta\ decays of ^{76}Ge are used as proxies for 0\nu\beta\beta\ decay. For BEGe detectors the chosen selection is based on a single pulse shape parameter. It accepts 0.92$\pm$0.02 of signal-like events while about 80% of the background events at Q_{\beta\beta}=2039 keV are rejected.
For semi-coaxial detectors three analyses are developed. The one based on an artificial neural network is used for the search of 0\nu\beta\beta\ decay. It retains 90% of DEP events and rejects about half of the events around Q_{\beta\beta}. The 2\nu\beta\beta\ events have an efficiency of 0.85\pm0.02 and the one for 0\nu\beta\beta\ decays is estimated to be 0.90^{+0.05}_{-0.09}. A second analysis uses a likelihood approach trained on Compton edge events. The third approach uses two pulse shape parameters. The latter two methods confirm the classification of the neural network since about 90% of the data events rejected by the neural network are also removed by both of them. In general, the selection efficiency extracted from DEP events agrees well with those determined from Compton edge events or from 2\nu\beta\beta\ decays.

M Agostini et al., (GERDA Collaboration), N. Frodyma, K. Pelczar, M. Wojcik, G. Zuzel

Download

http://arxiv.org/abs/1306.5084

http://link.springer.com/article/10.1140/epjc/s10052-014-2764-z

Abstract

The GERmanium Detector Array (GERDA) experiment at the Gran Sasso underground laboratory (LNGS) of INFN is searching for neutrinoless double beta decay of 76Ge. The signature of the signal is a monoenergetic peak at 2039 keV, the Q-value of the decay, Q_bb. To avoid bias in the signal search, the present analysis does not consider all those events, that fall in a 40 keV wide region centered around Q_bb. The main parameters needed for the neutrinoless double beta decay analysis are described. A background model was developed to describe the observed energy spectrum. The model contains several contributions, that are expected on the basis of material screening or that are established by the observation of characteristic structures in the energy spectrum. The model predicts a flat energy spectrum for the blinding window around Q_bb with a background index ranging from 17.6 to 23.8*10^{-3} counts/(keV kg yr). A part of the data not considered before has been used to test if the predictions of the background model are consistent. The observed number of events in this energy region is consistent with the background model. The background at Q-bb is dominated by close sources, mainly due to 42K, 214Bi, 228Th, 60Co and alpha emitting isotopes from the 226Ra decay chain. The individual fractions depend on the assumed locations of the contaminants. It is shown, that after removal of the known gamma peaks, the energy spectrum can be fitted in an energy range of 200 kev around Q_bb with a constant background. This gives a background index consistent with the full model and uncertainties of the same size.

M Agostini et al., (GERDA Collaboration), N. Frodyma, K. Pelczar, M. Wojcik, G. Zuzel

Download

http://dx.doi.org/10.1088/0954-3899/40/3/035110

Abstract

The primary goal of the GERmanium Detector Array (GERDA) experiment at the Laboratori Nazionali del Gran Sasso of INFN is the search for the neutrinoless double beta decay of 76Ge. High-purity germanium detectors made from material enriched in 76Ge are operated directly immersed in liquid argon, allowing for a substantial reduction of the background with respect to predecessor experiments. The first 5.04 kg yr of data collected in Phase I of the experiment have been analyzed to measure the half-life of the neutrino-accompanied double beta decay of 76Ge. The observed spectrum in the energy range between 600 and 1800 keV is dominated by the double beta decay of 76Ge. The half-life extracted from GERDA data is T2ν1/2 = (1.84+0.14−0.10) × 1021 yr.