This corrects the article DOI: 10.1103/PhysRevLett.125.011801.

The GERmanium Detector Array (Gerda) experiment at the Gran Sasso underground laboratory (LNGS) of INFN is searching for neutrinoless double-beta (0νββ) decay of 76Ge. The technological challenge of Gerda is to operate in a “background-free” regime in the region of interest (ROI) after analysis cuts for the full 100 kg·yr target exposure of the experiment. A careful modeling and decomposition of the full-range energy spectrum is essential to predict the shape and composition of events in the ROI around Qββ for the 0νββ search, to extract a precise measurement of the half-life of the double-beta decay mode with neutrinos (2νββ) and in order to identify the location of residual impurities. The latter will permit future experiments to build strategies in order to further lower the background and achieve even better sensitivities. In this article the background decomposition prior to analysis cuts is presented for Gerda Phase II. The background model fit yields a flat spectrum in the ROI with a background index (BI) of $$ {16.04}_{-0.85}^{+0.78}\cdotp {10}^{-3} $$ cts/(keV·kg·yr) for the enriched BEGe data set and $$ {14.68}_{-0.52}^{+0.47}\cdotp {10}^{-3} $$ cts/(keV·kg·yr) for the enriched coaxial data set. These values are similar to the one of Phase I despite a much larger number of detectors and hence radioactive hardware components.

We present the first search for bosonic superweakly interacting massive particles (super-WIMPs) as keV-scale dark matter candidates performed with the GERDA experiment. GERDA is a neutrinoless double-β decay experiment which operates high-purity germanium detectors enriched in ^{76}Ge in an ultralow background environment at the Laboratori Nazionali del Gran Sasso (LNGS) of INFN in Italy. Searches were performed for pseudoscalar and vector particles in the mass region from 60  keV/c^{2} to 1  MeV/c^{2}. No evidence for a dark matter signal was observed, and the most stringent constraints on the couplings of super-WIMPs with masses above 120  keV/c^{2} have been set. As an example, at a mass of 150  keV/c^{2} the most stringent direct limits on the dimensionless couplings of axionlike particles and dark photons to electrons of g_{ae}<3×10^{-12} and α^{'}/α<6.5×10^{-24} at 90% credible interval, respectively, were obtained.

The GERmanium Detector Array (GERDA) experiment searched for the lepton-number-violating neutrinoless double-β (0νββ) decay of ^{76}Ge, whose discovery would have far-reaching implications in cosmology and particle physics. By operating bare germanium diodes, enriched in ^{76}Ge, in an active liquid argon shield, GERDA achieved an unprecedently low background index of 5.2×10^{-4} counts/(keV kg yr) in the signal region and met the design goal to collect an exposure of 100 kg yr in a background-free regime. When combined with the result of Phase I, no signal is observed after 127.2 kg yr of total exposure. A limit on the half-life of 0νββ decay in ^{76}Ge is set at T_{1/2}>1.8×10^{26}  yr at 90% C.L., which coincides with the sensitivity assuming no signal.

A discovery that neutrinos are Majorana fermions would have profound implications for particle physics and cosmology. The Majorana character of neutrinos would make possible the neutrinoless double-β (0νββ) decay, a matter-creating process without the balancing emission of antimatter. The GERDA Collaboration searches for the 0νββ decay of 76Ge by operating bare germanium detectors in an active liquid argon shield. With a total exposure of 82.4 kg⋅year, we observe no signal and derive a lower half-life limit of T 1/2 > 0.9 × 1026 years (90% C.L.). Our T 1/2 sensitivity, assuming no signal, is 1.1 × 1026 years. Combining the latter with those from other 0νββ decay searches yields a sensitivity to the effective Majorana neutrino mass of 0.07 to 0.16 electron volts.

The GERmanium Detector Array (Gerda) is a low background experiment located at the Laboratori Nazionali del Gran Sasso in Italy, which searches for neutrinoless double-beta decay of $$^{76}$$Ge into $$^{76}$$Se+2e$$^-$$. Gerda has been conceived in two phases. Phase II, which started in December 2015, features several novelties including 30 new 76Ge enriched detectors. These were manufactured according to the Broad Energy Germanium (BEGe) detector design that has a better background discrimination capability and energy resolution compared to formerly widely-used types. Prior to their installation, the new BEGe detectors were mounted in vacuum cryostats and characterized in detail in the Hades underground laboratory in Belgium. This paper describes the properties and the overall performance of these detectors during operation in vacuum. The characterization campaign provided not only direct input for Gerda Phase II data collection and analyses, but also allowed to study detector phenomena, detector correlations as well as to test the accuracy of pulse shape simulation codes.

The GERDA experiment searches for the lepton-number-violating neutrinoless double-β decay of ^{76}Ge (^{76}Ge→^{76}Se+2e^{-}) operating bare Ge diodes with an enriched ^{76}Ge fraction in liquid argon. The exposure for broad-energy germanium type (BEGe) detectors is increased threefold with respect to our previous data release. The BEGe detectors feature an excellent background suppression from the analysis of the time profile of the detector signals. In the analysis window a background level of 1.0_{-0.4}^{+0.6}×10^{-3}  counts/(keV kg yr) has been achieved; if normalized to the energy resolution this is the lowest ever achieved in any 0νββ experiment. No signal is observed and a new 90% C.L. lower limit for the half-life of 8.0×10^{25}  yr is placed when combining with our previous data. The expected median sensitivity assuming no signal is 5.8×10^{25}  yr.

The GERDA collaboration is performing a sensitive search for neutrinoless double beta decay of ^76Ge 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.

M. Agostini, b A.M. Bakalyarov, M. Balata, I. Barabanov, L. Baudis, C. Bauer, E. Bellotti, 10 S. Belogurov, 12, c A. Bettini, 18 L. Bezrukov, J. Biernat, T. Bode, D. Borowicz, d V. Brudanin, R. Brugnera, 18 A. Caldwell, C. Cattadori, A. Chernogorov, T. Comellato, V. D’Andrea, E.V. Demidova, N. Di Marco, A. Domula, E. Doroshkevich, V. Egorov, R. Falkenstein, A. Gangapshev, 8 A. Garfagnini, 18 P. Grabmayr, V. Gurentsov, K. Gusev, 14, 16 J. Hakenmüller, A. Hegai, M. Heisel, S. Hemmer, R. Hiller, W. Hofmann, M. Hult, L.V. Inzhechik, e J. Janicskó Csáthy, f J. Jochum, M. Junker, V. Kazalov, Y. Kermaidic, T. Kihm, I.V. Kirpichnikov, A. Kirsch, A. Kish, A. Klimenko, 6 R. Kneißl, K.T. Knöpfle, O. Kochetov, V.N. Kornoukhov, 12 V.V. Kuzminov, M. Laubenstein, A. Lazzaro, M. Lindner, I. Lippi, A. Lubashevskiy, B. Lubsandorzhiev, G. Lutter, C. Macolino, g B. Majorovits, W. Maneschg, M. Miloradovic, R. Mingazheva, M. Misiaszek, P. Moseev, I. Nemchenok, K. Panas, L. Pandola, K. Pelczar, L. Pertoldi, 18 A. Pullia, C. Ransom, S. Riboldi, N. Rumyantseva, 6 C. Sada, 18 F. Salamida, C. Schmitt, B. Schneider, S. Schönert, A-K. Schütz, O. Schulz, B. Schwingenheuer, O. Selivanenko, E. Shevchik, M. Shirchenko, H. Simgen, A. Smolnikov, 6 L. Stanco, L. Vanhoefer, A.A. Vasenko, A. Veresnikova, K. von Sturm, 18 V. Wagner, h A. Wegmann, T. Wester, C. Wiesinger, M. Wojcik, E. Yanovich, I. Zhitnikov, S.V. Zhukov, D. Zinatulina, A. Zschocke, A.J. Zsigmond, K. Zuber, and G. Zuzel INFN Laboratori Nazionali del Gran Sasso and Gran Sasso Science Institute, Assergi, Italy INFN Laboratori Nazionali del Gran Sasso and Università degli Studi dell’Aquila, L’Aquila, Italy INFN Laboratori Nazionali del Sud, Catania, Italy Institute of Physics, Jagiellonian University, Cracow, Poland Institut für Kernund Teilchenphysik, Technische Universität Dresden, Dresden, Germany Joint Institute for Nuclear Research, Dubna, Russia European Commission, JRC-Geel, Geel, Belgium Max-Planck-Institut für Kernphysik, Heidelberg, Germany Dipartimento di Fisica, Università Milano Bicocca, Milan, Italy INFN Milano Bicocca, Milan, Italy Dipartimento di Fisica, Università degli Studi di Milano e INFN Milano, Milan, Italy Institute for Nuclear Research of the Russian Academy of Sciences, Moscow, Russia Institute for Theoretical and Experimental Physics, NRC “Kurchatov Institute”, Moscow, Russia National Research Centre “Kurchatov Institute”, Moscow, Russia Max-Planck-Institut für Physik, Munich, Germany Physik Department and Excellence Cluster Universe, Technische Universität München, Germany Dipartimento di Fisica e Astronomia dell‘Università di Padova, Padua, Italy INFN Padova, Padua, Italy Physikalisches Institut, Eberhard Karls Universität Tübingen, Tübingen, Germany Physik Institut der Universität Zürich, Zurich, Switzerland

The GERmanium Detector Array (GERDA) is a low background experiment at the Laboratori Nazionali del Gran Sasso (LNGS) of INFN designed to search for the rare neutrinoless double beta decay ([Formula: see text]) of [Formula: see text]Ge. In the first phase (Phase I) of the experiment, high purity germanium diodes were operated in a “bare” mode and immersed in liquid argon. The overall background level of [Formula: see text] was a factor of ten better than those of its predecessors. No signal was found and a lower limit was set on the half-life for the [Formula: see text] decay of [Formula: see text]Ge [Formula: see text] yr (90% CL), while the corresponding median sensitivity was [Formula: see text] yr (90% CL). A second phase (Phase II) started at the end of 2015 after a major upgrade. Thanks to the increased detector mass and performance of the enriched germanium diodes and due to the introduction of liquid argon instrumentation techniques, it was possible to reduce the background down to [Formula: see text]. After analyzing 23.2 kg[Formula: see text]⋅[Formula: see text]yr of these new data no signal was seen. Combining these with the data from Phase I a stronger half-life limit of the [Formula: see text]Ge [Formula: see text] decay was obtained: [Formula: see text] yr (90% CL), reaching a sensitivity of [Formula: see text] yr (90% CL). Phase II will continue for the collection of an exposure of 100 kg[Formula: see text]yr. If no signal is found by then the GERDA sensitivity will have reached [Formula: see text] yr for setting a 90% CL. limit. After the end of GERDA Phase II, the flagship experiment for the search of [Formula: see text] decay of [Formula: see text]Ge will be LEGEND. LEGEND experiment is foreseen to deploy up to 1-ton of [Formula: see text]Ge. After ten years of data taking, it will reach a sensitivity beyond 10[Formula: see text] yr, and hence fully cover the inverted hierarchy region.

The Gerda experiment, located at the Laboratori Nazionali del Gran Sasso (LNGS) of INFN in Italy, searches for the neutrinoless double beta (0νββ) decay of 76Ge. Gerda Phase II is aiming to reach a sensitivity for the 0νββ half life of 1026 yr in ∼ 3 years of physics data taking with 100 kg·yr of exposure and a background index of ∼ 10−3 cts/(keV·kg·yr). After 6 months of acquisition a first data release with 10.8 kg·yr of exposure is performed, showing that the design background is achieved. In this work a study of the Phase II background spectrum, the main spectral structures and the background sources will be presented and discussed.

GERDA is designed for a background-free search of Ge-76 neutrinoless double-beta decay, using bare Ge detectors in liquid Ar. The experiment was upgraded after the successful completion of Phase I to double the target mass and further reduce the background. Newly-designed Ge detectors were installed along with LAr scintillation sensors. Phase II of data-taking started in Dec 2015 with approximately 36 kg of Ge detectors and is currently ongoing. The first results based on 10.8kg.yr of exposure are presented. The background goal of 10(-3) cts/(keV.kg.yr) is achieved and a search for neutrinoless double-beta decay is performed by combining Phase I and II data. No signal is found and a new limit is set at T-1/2(0 nu) >5.3 . 10(25) yr (90% C.L.).

The GERDA (GERmanium Detector Array) is an experiment for the search of neutrinoless double beta decay (0v beta) in Ge-76, located at Laboratori Nazionali del Gran Sasso of INFN (Italy). GERDA operates bare high purity germanium detectors submersed in liquid Argon (LAr). Phase II of data-taking started in Dec 2015 and is currently ongoing. In Phase II 35 kg of germanium detectors enriched in Ge-76 including thirty newly produced Broad Energy Germanium (BEGe) detectors is operating to reach an exposure of 100 kg.yr within about 3 years data taking. The design goal of Phase II is to reduce the background by one order of magnitude to get the sensitivity for T-1/2(0v) = O(10(26)) yr. To achieve the necessary background reduction, the setup was complemented with LAr veto. Analysis of the background spectrum of Phase II demonstrates consistency with the background models. Furthermore Ra-226 and Th-232 contamination levels consistent with screening results. In the first Phase II data release we found no hint for a Ov beta beta decay signal and place a limit of this process T-1/2(0v) > 5.3.10(25) yr (90% C.L., sensitivity 4.0.10(25) yr). First results of GERDA Phase II will be presented.

Two neutrino double beta decay of 76Ge to excited states of 76Se 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 76Ge 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 DOI: https://doi.org/10.1088/0954-3899/42/11/115201 Posted at the Zurich Open Repository and Archive, University of Zurich ZORA URL: https://doi.org/10.5167/uzh-122053 Accepted Version Originally published at: GERDA Collaboration; Agostini, M; Allardt, M; Bakalyarov, A M; Baudis, L; Benato, G; Walter, M; et al (2015). 2νββ decay of 76Ge into excited states with GERDA Phase I. Journal of Physics G: Nuclear and Particle Physics, 42(11):115201-115218. DOI: https://doi.org/10.1088/0954-3899/42/11/115201 2νββ decay of Ge into excited states with Gerda Phase I Gerda Collaboration‡ M. Agostini, M. Allardt, A.M. Bakalyarov, M. Balata, I. Barabanov, N. Barros§, L. Baudis, C. Bauer, N. Becerici-Schmidt, E. Bellotti, S. Belogurov, S.T. Belyaev, G. Benato, A. Bettini, L. Bezrukov, T. Bode, D. Borowicz, V. Brudanin, R. Brugnera, D. Budjáš, A. Caldwell, C. Cattadori, A. Chernogorov, V. D’Andrea, E.V. Demidova, A. di Vacri, A. Domula, E. Doroshkevich, V. Egorov, R. Falkenstein, O. Fedorova, K. Freund, N. Frodyma, A. Gangapshev, A. Garfagnini, C. Gooch, P. Grabmayr, V. Gurentsov, K. Gusev, A. Hegai, M. Heisel, S. Hemmer, G. Heusser, W. Hofmann, M. Hult, L.V. Inzhechik‖, J. Janicskó Csáthy, J. Jochum, M. Junker, V. Kazalov, T. Kihm, I.V. Kirpichnikov, A. Kirsch, A. Klimenko¶, K.T. Knöpfle, O. Kochetov, V.N. Kornoukhov, V.V. Kuzminov, M. Laubenstein, A. Lazzaro, V.I. Lebedev, B. Lehnert, H.Y. Liao, M. Lindner, I. Lippi, A. Lubashevskiy, B. Lubsandorzhiev, G. Lutter, C. Macolino, B. Majorovits, W. Maneschg, E. Medinaceli, Y. Mi, M. Misiaszek, P. Moseev, I. Nemchenok, D. Palioselitis, K. Panas, L. Pandola, K. Pelczar, A. Pullia, S. Riboldi, N. Rumyantseva, C. Sada, M. Salathe, C. Schmitt, B. Schneider, J. Schreiner, O. Schulz, B. Schwingenheuer, S. Schönert, A-K. Schütz, O. Selivanenko, M. Shirchenko, H. Simgen, A. Smolnikov, L. Stanco, M. Stepaniuk, C.A. Ur, L. Vanhoefer, A.A. Vasenko, A. Veresnikova, K. von Sturm, V. Wagner, M. Walter, A. Wegmann, ‡ LNGS, Assergi, Italy; correspondence: gerda-eb@mpi-hd.mpg.de § present address: Dept. of Physics and Astronomy, Univ. of Pennsylvania, Philadelphia, Pennsylvania, USA ‖ also at: Moscow Inst. of Physics and Technology, Russia ¶ also at: Int. Univ. for Nature, Society and Man “Dubna”, Dubna, Russia ar X iv :1 50 6. 03 12 0v 1 [ he pex ] 9 J un 2 01 5 2νββ decay of Ge into excited states with Gerda Phase I 2 T. Wester, H. Wilsenach, M. Wojcik, E. Yanovich, P. Zavarise, I. Zhitnikov, S.V. Zhukov, D. Zinatulina, K. Zuber, and G. Zuzel. 1 INFN Laboratori Nazionali del Gran Sasso and Gran Sasso Science Institute, Assergi, Italy 2 INFN Laboratori Nazionali del Sud, Catania, Italy 3 Institute of Physics, Jagiellonian University, Cracow, Poland 4 Institut für Kernund Teilchenphysik, Technische Universität Dresden, Dresden, Germany 5 Joint Institute for Nuclear Research, Dubna, Russia 6 Institute for Reference Materials and Measurements, Geel, Belgium 7 Max-Planck-Institut für Kernphysik, Heidelberg, Germany 8 Dipartimento di Fisica, Università Milano Bicocca, Milano, Italy 9 INFN Milano Bicocca, Milano, Italy 10 Dipartimento di Fisica, Università degli Studi di Milano e INFN Milano, Milano, Italy 11 Institute for Nuclear Research of the Russian Academy of Sciences, Moscow, Russia 12 Institute for Theoretical and Experimental Physics, Moscow, Russia 13 National Research Centre “Kurchatov Institute”, Moscow, Russia 14 Max-Planck-Institut für Physik, München, Germany 15 Physik Department and Excellence Cluster Universe, Technische Universität München, Germany 16 Dipartimento di Fisica e Astronomia dell‘Università di Padova, Padova, Italy 17 INFN Padova, Padova, Italy 18 Physikalisches Institut, Eberhard Karls Universität Tübingen, Tübingen, Germany 19 Physik Institut der Universität Zürich, Zürich, Switzerland Abstract. Two neutrino double beta decay of Ge to excited states of 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 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: 0g.s. − 21 : T 2ν 1/2> 1.6·10 23 yr, 0g.s. − 01 : T 2ν 1/2> 3.7·10 23 yr and 0g.s. − 22 : T 2ν 1/2> 2.3·10 23 yr. These bounds are more than two orders of magnitude larger than those reported previously. Bayesian 90 % credibility bounds were extracted and used to exclude several models for the 0g.s. − 01 transition. Two neutrino double beta decay of Ge to excited states of 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 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: 0g.s. − 21 : T 2ν 1/2> 1.6·10 23 yr, 0g.s. − 01 : T 2ν 1/2> 3.7·10 23 yr and 0g.s. − 22 : T 2ν 1/2> 2.3·10 23 yr. These bounds are more than two orders of magnitude larger than those reported previously. Bayesian 90 % credibility bounds were extracted and used to exclude several models for the 0g.s. − 01 transition. PACS numbers: 23.40.-s, 21.10.Tg, 27.50.+e, 29.40.Wk

The observation of neutrinoless double-beta decay (0 nu beta beta) would show that lepton number is violated, reveal that neutrinos are Majorana particles, and provide information on neutrino mass. A discovery-capable experiment covering the inverted ordering region, with effective Majorana neutrino masses of 15 - 50 meV, will require a tonne-scale experiment with excellent energy resolution and extremely low backgrounds, at the level of similar to 0.1 count/(FWHM.t.yr) in the region of the signal. The current generation Ge-76 experiments GERDA and the Majorana Demonstrator, utilizing high purity Germanium detectors with an intrinsic energy resolution of 0.12%, have achieved the lowest backgrounds by over an order of magnitude in the 0 nu beta beta signal region of all 0 nu beta beta experiments. Building on this success, the LEGEND collaboration has been formed to pursue a tonne-scale Ge-76 experiment. The collaboration aims to develop a phased 0 nu beta beta experimental program with discovery potential at a half-life approaching or at 1028 years, using existing resources as appropriate to expedite physics results.

The observation of neutrinoless double beta decay would allow to shed light onto the particle nature of neutrinos. Gerda is aiming to perform a background-free search for this process using high purity germanium detectors enriched in 76Ge operated in liquid argon. This goal relies on the application of active background suppression techniques. A low background light instrumentation has been installed for Phase II to detect events with coincident energy deposition in the nearby liquid argon. The intended background index of ∼10−3 cts/(keV·ky·yr) has been confirmed.

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.

. Gerda is designed for a background-free search of 76 Ge neutrinoless double- β decay, using bare Ge detectors in liquid Ar. The experiment was upgraded after the successful completion of PhaseI to double the target mass and further reduce the background. Newly-designed Ge detectors were installed along with LAr scintillation sensors. PhaseII of data-taking started in Dec 2015 with approximately 36 kg of Ge detectors and is currently ongoing. The first results based on 10 . 8kg · yr of exposure are presented. The background goal of 10 − 3 cts / (keV · kg · yr) is achieved and a search for neutrinoless double- β decay is performed by combining PhaseI and II data. No signal is found and a new limit is set at T 0 ν 1 / 2 > 5 . 3 · 10 25 yr (90% C.L.).