Commercial argon contains cosmogenic ^42Ar whose progeny ^42K is a critical background component for the Large Enriched Germanium Experiment for Neutrinoless ββ Decay (LEGEND). LEGEND operates High-Purity Germanium (HPGe) detectors bare in liquid argon. ^42K is attracted by the HPGe detectors' electric fields, and drifts toward the germanium surface, where it undergoes beta decay. LEGEND-1000 will mitigate ^42K-induced background by using underground-sourced argon, depleted in cosmogenic isotopes. If underground argon is not available, mitigation techniques must be employed. Poly(ethylene naphthalate) (PEN) enclosures were proposed to hinder the ion drift, decrease the beta-particle's energy, and produce scintillation light. In this paper, we report on operating two HPGe detectors, both bare and PEN-enclosed, in ^42Ar-enriched liquid argon, and find no evidence for deterioration of energy stability or resolution due to the enclosures. We monitor the beta and gamma rates of ^42K, find complex time-dependencies extending to roughly 30 days after applying the HPGe detectors' high-voltage, and qualitatively demonstrate the ^42K suppression capabilities of enclosures.

LEGEND-200 is an experiment designed to search for neutrinoless double beta decay of Ge-76 by operating up to 200 kg of enriched germanium detectors in liquid argon (LAr). To achieve ultra-low backgrounds, the LAr is instrumented to detect scintillation light emitted upon interactions with ionizing radiation, thus tagging and rejecting backgrounds. The LAr scintillation light is detected with wavelength-shifting fibers coupled to silicon photomultiplier (SiPM) arrays. In this document, we demonstrate the high photoelectron resolution and low noise level of the SiPM signals. We also present the results of special calibration runs performed to determine the light yield and background suppression factors. We show the background suppression performance of the LAr instrumentation on the LEGEND-200 background spectrum before and after the LAr light coincidence cut. Finally, we present the event topology classifier which enables effective particle discrimination, allowing the identification of background types in LEGEND-200.

A technique for producing, adjusting, and testing large-area scintillation detectors (≥1 m2) for active cosmic muon shielding in low-background experiments is presented. The detectors developed feature a high efficiency ( 99.75

A search for full energy depositions from bosonic keV-scale dark matter candidates of masses between 65 keV and 1021 keV has been performed with data collected during Phase II of the GERmanium Detector Array (GERDA) experiment. Our analysis includes direct dark matter absorption as well as dark Compton scattering. With a total exposure of 105.5 kg yr, no evidence for a signal above the background has been observed. The resulting exclusion limits deduced with either Bayesian or Frequentist statistics are the most stringent direct constraints in the major part of the 140-1021 keV mass range. As an example, at a mass of 150 keV the dimensionless coupling of dark photons and axion-like particles to electrons has been constrained to $\alpha$'/$\alpha$ < 8.7x10$^{-24}$ and g$_{ae}$ < 3.3x10$^{-12}$ at 90% credible interval (CI), respectively. Additionally, a search for peak-like signals from beyond the Standard Model decays of nucleons and electrons is performed. We find for the inclusive decay of a single neutron in $^{76}$Ge a lower lifetime limit of $\tau_n$ > 1.5x10$^{24}$ yr and for a proton $\tau_p$ > 1.3x10$^{24}$ yr at 90% CI. For the electron decay e$^-\rightarrow\nu_e\gamma$ a lower limit of $\tau_e$ > 5.4x10$^{25}$ yr at 90% CI has been determined.

Abstract The Monument experiment measures ordinary muon capture (OMC) on isotopes relevant for neutrinoless double-beta ( $$0\nu \beta \beta $$ 0 ν β β ) decay and nuclear astrophysics. OMC is a particularly attractive tool for improving the theoretical description of $$0\nu \beta \beta $$ 0 ν β β decay. It involves similar momentum transfers and allows testing the virtual transitions involved in $$0\nu \beta \beta $$ 0 ν β β decay against experimental data. During the 2021 campaign, Monument studied OMC on $$^{76}$$ 76 Se and $$^{136}$$ 136 Ba, the isotopes relevant for next-generation $$0\nu \beta \beta $$ 0 ν β β decay searches, like Legend and nEXO. The experimental setup has been designed to accurately extract the total and partial muon capture rates, which requires precise reconstruction of energies and time-dependent intensities of the OMC-related $$\gamma $$ γ rays. The setup also includes a veto counter system to allow selecting a clean sample of OMC events. This work provides a detailed description of the Monument setup operated during the 2021 campaign, its two DAQ systems, calibration and analysis approaches, and summarises the achieved detector performance. Future improvements are also discussed.

The Monument experiment measures ordinary muon capture (OMC) on isotopes relevant for neutrinoless double-beta ( 0νββ ) decay and nuclear astrophysics. OMC is a particularly attractive tool for improving the theoretical description of 0νββ decay. It involves similar momentum transfers and allows testing the virtual transitions involved in 0νββ decay against experimental data. During the 2021 campaign, Monument studied OMC on ^76 Se and ^136 Ba, the isotopes relevant for next-generation 0νββ decay searches, like Legend and nEXO. The experimental setup has been designed to accurately extract the total and partial muon capture rates, which requires precise reconstruction of energies and time-dependent intensities of the OMC-related γ rays. The setup also includes a veto counter system to allow selecting a clean sample of OMC events. This work provides a detailed description of the Monument setup operated during the 2021 campaign, its two DAQ systems, calibration and analysis approaches, and summarises the achieved detector performance. Future improvements are also discussed.

The ability to detect liquid argon scintillation light from within a densely packed high-purity germanium detector array allowed the GERDA experiment to reach an exceptionally low background rate in the search for neutrinoless double beta decay of ^76Ge. Proper modeling of the light propagation throughout the experimental setup, from any origin in the liquid argon volume to its eventual detection by the novel light read-out system, provides insight into the rejection capability and is a necessary ingredient to obtain robust background predictions. In this paper, we present a model of the GERDA liquid argon veto, as obtained by Monte Carlo simulations and constrained by calibration data, and highlight its application for background decomposition.

We present the measurement of the two-neutrino double-β decay rate of ^{76}Ge performed with the GERDA Phase II experiment. With a subset of the entire GERDA exposure, 11.8 kg yr, the half-life of the process has been determined: T_{1/2}^{2ν}=(2.022±0.018_{stat}±0.038_{syst})×10^{21}  yr. This is the most precise determination of the ^{76}Ge two-neutrino double-β decay half-life and one of the most precise measurements of a double-β decay process. The relevant nuclear matrix element can be extracted: M_{eff}^{2ν}=(0.101±0.001).

Abstract We search for tri-nucleon decays of $$^{76}$$ 76 Ge in the dataset from the GERmanium Detector Array (GERDA) experiment. Decays that populate excited levels of the daughter nucleus above the threshold for particle emission lead to disintegration and are not considered. The ppp-, ppn-, and pnn-decays lead to $$^{73}$$ 73 Cu, $$^{73}$$ 73 Zn, and $$^{73}$$ 73 Ga nuclei, respectively. These nuclei are unstable and eventually proceed by the beta decay of $$^{73}$$ 73 Ga to $$^{73}$$ 73 Ge (stable). We search for the $$^{73}$$ 73 Ga decay exploiting the fact that it dominantly populates the 66.7 keV $$^{73m}$$ 73 m Ga state with half-life of 0.5 s. The nnn-decays of $$^{76}$$ 76 Ge that proceed via $$^{73m}$$ 73 m Ge are also included in our analysis. We find no signal candidate and place a limit on the sum of the decay widths of the inclusive tri-nucleon decays that corresponds to a lower lifetime limit of 1.2 $$\times $$ × 10 $$^{26}$$ 26 yr (90% credible interval). This result improves previous limits for tri-nucleon decays by one to three orders of magnitude.

We search for tri-nucleon decays of ^76 Ge in the dataset from the GERmanium Detector Array (GERDA) experiment. Decays that populate excited levels of the daughter nucleus above the threshold for particle emission lead to disintegration and are not considered. The ppp-, ppn-, and pnn-decays lead to ^73 Cu, ^73 Zn, and ^73 Ga nuclei, respectively. These nuclei are unstable and eventually proceed by the beta decay of ^73 Ga to ^73 Ge (stable). We search for the ^73 Ga decay exploiting the fact that it dominantly populates the 66.7 keV ^73m Ga state with half-life of 0.5 s. The nnn-decays of ^76 Ge that proceed via ^73m Ge are also included in our analysis. We find no signal candidate and place a limit on the sum of the decay widths of the inclusive tri-nucleon decays that corresponds to a lower lifetime limit of 1.2 × 10 ^26 yr (90

Abstract The GERmanium Detector Array (Gerda) collaboration searched for neutrinoless double- $$\beta $$ β decay in $$^{76}$$ 76 Ge using isotopically enriched high purity germanium detectors at the Laboratori Nazionali del Gran Sasso of INFN. After Phase I (2011–2013), the experiment benefited from several upgrades, including an additional active veto based on LAr instrumentation and a significant increase of mass by point-contact germanium detectors that improved the half-life sensitivity of Phase II (2015–2019) by an order of magnitude. At the core of the background mitigation strategy, the analysis of the time profile of individual pulses provides a powerful topological discrimination of signal-like and background-like events. Data from regular $$^{228}$$ 228 Th calibrations and physics data were both considered in the evaluation of the pulse shape discrimination performance. In this work, we describe the various methods applied to the data collected in Gerda Phase II corresponding to an exposure of 103.7 kg year. These methods suppress the background by a factor of about 5 in the region of interest around $$Q_{\beta \beta }= 2039$$ Q β β = 2039 keV, while preserving $$(81\pm 3)$$ ( 81 ± 3 ) % of the signal. In addition, an exhaustive list of parameters is provided which were used in the final data analysis.

A description of an experiment devoted to determining the total capture rates of negative muons in 24 Mg is presented. The investigations are carried out using a low-energy muon beam with high intensity (10 4 s –1 ) at the SμS meson facility in Villigen, Switzerland, with the help of the mini-Ball cluster of HPGe detectors. The experimental approach based on our previous works is described. The justification of the difference between these methods and those used in previous investigations is presented. It is pointed out that our method is potentially much more precise and does not depend on the muon absorption in the surrounding matter. A comparison of these results with those of previous experiments and theoretical calculations is also presented.

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

Poly Ethylene Naphthalate (PEN) is an industrial polymer plastic which is investigated as a low background, transparent, scintillating and wavelength shifting structural material. PEN scintillates in the blue region and has excellent mechanical properties both at room and cryogenic temperatures. Thus, it is an ideal candidate for active structural components in experiments for the search of rare events like neutrinoless double-beta decay or dark matter recoils. Such optically active structures improve the identification and rejection efficiency of backgrounds events, like this improving the sensitivity of experiments. This paper reports on the production of radiopure and transparent PEN plates These structures can be used to mount germanium detectors operating in cryogenic liquids (LAr, LN). Thus, as first application PEN holders will be used to mount the Ge detectors in the LEGEND-200 experiment. The whole process from cleaning the raw material to testing the PEN active components under final operational conditions is reported.

A search for Beyond the Standard Model double- β decay modes of 76 Ge has been performed with data collected during the Phase II of the GERmanium Detector Array ( Gerda ) experiment, located at the Laboratori Nazionali del Gran Sasso of INFN (Italy). Improved limits on the decays involving Majorons have been obtained, compared to previous experiments with 76 Ge, with half-life values on the order of 10 23 yr. For the first time with 76 Ge, limits on Lorentz invariance violation effects in double- β decay have been obtained. The isotropic coefficient å of (3) , which embeds Lorentz violation in double- β decay, has been constrained at the order of 10 -6 GeV. We also set the first experimental limits on the search for light exotic fermions in double- β decay, including sterile neutrinos.

We propose the construction of LEGEND-1000, the ton-scale Large Enriched Germanium Experiment for Neutrinoless $\beta \beta$ Decay. This international experiment is designed to answer one of the highest priority questions in fundamental physics. It consists of 1000 kg of Ge detectors enriched to more than 90% in the $^{76}$Ge isotope operated in a liquid argon active shield at a deep underground laboratory. By combining the lowest background levels with the best energy resolution in the field, LEGEND-1000 will perform a quasi-background-free search and can make an unambiguous discovery of neutrinoless double-beta decay with just a handful of counts at the decay $Q$ value. The experiment is designed to probe this decay with a 99.7%-CL discovery sensitivity in the $^{76}$Ge half-life of $1.3\times10^{28}$ years, corresponding to an effective Majorana mass upper limit in the range of 9-21 meV, to cover the inverted-ordering neutrino mass scale with 10 yr of live time.

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.

LEGEND is the next-generation experiment searching for the neutrinoless double beta decay in 76Ge. The first stage, LEGEND-200, takes over the cryogenic infrastructure of GERDA at LNGS: an instrumented water tank surrounding a 64 m3 liquid argon cryostat. Around 200 kg of Ge detectors will be deployed in the cryostat, with the liquid argon acting as cooling medium, high-purity passive shielding and secondary detection medium. For the latter purpose, a liquid argon instrumentation is developed, based on the system used in GERDA Phase II. Wavelength shifting fibers coated with TPB are arranged in two concentric barrels. Both ends are read out by SiPM arrays. A wavelength shifting reflector surrounds the array in order to enhance the light collection far from the array. The LLAMA is installed in the cryostat to permanently monitor the optical parameters and to provide in-situ inputs for modeling purposes. The design of all parts of the LEGEND-200 LAr instrumentation is presented. An overview of the geometry, operation principle, and off-line data analysis of the LLAMA is shown.