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The European Research Council has recently funded HOLMES, a new experiment to directly measure the neutrino mass. HOLMES will perform a calorimetric measurement of the energy released in the decay of [Formula: see text]Ho. The calorimetric measurement eliminates systematic uncertainties arising from the use of external beta sources, as in experiments with beta spectrometers. This measurement was proposed in 1982 by A. De Rujula and M. Lusignoli, but only recently the detector technological progress allowed to design a sensitive experiment. HOLMES will deploy a large array of low temperature microcalorimeters with implanted [Formula: see text]Ho nuclei. The resulting mass sensitivity will be as low as 0.4 eV. HOLMES will be an important step forward in the direct neutrino mass measurement with a calorimetric approach as an alternative to spectrometry. It will also establish the potential of this approach to extend the sensitivity down to 0.1 eV. We outline here the project with its technical challenges and perspectives.

Neutrinoless double beta decay ( 0 ν β β ) is one of the most sensitive probes for physics beyond the Standard Model, providing unique information on the nature of neutrinos. In this paper we review the status and outlook for bolometric 0 ν β β  decay searches. We summarize recent advances in background suppression demonstrated using bolometers with simultaneous readout of heat and light signals. We simulate several configurations of a future CUORE-like bolometer array which would utilize these improvements and present the sensitivity reach of a hypothetical next-generation bolometric 0 ν β β  experiment. We demonstrate that a bolometric experiment with the isotope mass of about 1 ton is capable of reaching the sensitivity to the effective Majorana neutrino mass ( | m e e | ) of order 10–20 meV, thus completely exploring the so-called inverted neutrino mass hierarchy region. We highlight the main challenges and identify priorities for an R&D program addressing them.

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.

The GERmanium Detector Array ( Gerda ) experiment operated enriched high-purity germanium detectors in a liquid argon cryostat, which contains 0.33% of$$^{36}$$36 Ar, a candidate isotope for the two-neutrino double-electron capture (2$$\\nu $$ν ECEC) and therefore for the neutrinoless double-electron capture (0$$\\nu $$ν ECEC). If detected, this process would give evidence of lepton number violation and the Majorana nature of neutrinos. In the radiative 0$$\\nu $$ν ECEC of$$^{36}$$36 Ar, a monochromatic photon is emitted with an energy of 429.88 keV, which may be detected by the Gerda germanium detectors. We searched for the$$^{36}$$36 Ar 0$$\\nu $$ν ECEC with Gerda data, with a total live time of 4.34 year (3.08 year accumulated during Gerda Phase II and 1.26 year during Gerda Phase I). No signal was found and a 90% CL lower limit on the half-life of this process was established$$T_{1/2} >1.5\\cdot 10^{22} $$T 1 / 2 > 1.5 · 10 22  year.

(ProQuest: ... denotes formulae and/or non-USASCII text omitted; see image).The GERmanium Detector Array (Gerda) at the Gran Sasso Underground Laboratory (LNGS) searches for the neutrinoless double beta decay (...) of ...Ge. Germanium detectors made of material with an enriched ...Ge 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 ...Ge 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 ...Ge 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.

This paper describes the production and chemical separation of the 163Ho isotope that will be used in several nuclear physics experiments aiming at measuring the neutrino mass as well as the neutron cross section of the 163Ho isotope. For this purpose, several batches of enriched 162Er have been irradiated at the Institut Laue-Langevin high flux reactor to finally produce 6 mg or 100 MBq of the desired 163Ho isotope. A portion of the Er/Ho mixture is then subjected to a sophisticated chemical separation involving ion exchange chromatography to isolate the Ho product from the Er target material. Before irradiation, a thorough analysis of the impurity content was performed and its implication on the produced nuclide inventory will be discussed.

The assessment of the absolute ν mass scale is a crucial challenge in today’s particle physics and cosmology. The only experimental method which can provide a model-independent measurement is the investigation of endpoint distortion in beta/electron capture spectra. 163 Ho is a good choice thanks to its low electron capture Q value (about 2.8 keV), the proximity of the end-point to resonance M1 and its half-life (4570 years). The HOLMES experiment will exploit a calorimetric measurement of 163 Ho decay spectrum deploying a large set of cryogenic micro-calorimeters containing implanted 163 Ho. In order to get the best experimental sensitivity, it is crucial to combine high activity with very small undetected pileup contribution. Therefore, the main tasks of the experiment consist of: the development of about 1000 fast (3 μ s time resolution) cryogenic micro-calorimeters characterized by extraordinary energy resolution (down to few eV); the embedding of 163 Ho source inside the calorimeters, avoiding to spoil detectors’ thermodynamical properties (mainly heat capacity) and preventing pileup issues. Moreover, it is also necessary to avoid contamination from other radionuclides, mainly 166 m Ho. Finally, an efficient high-bandwidth multiplexed readout has to be developed. The commissioning of the first implanted array is currently ongoing; the first data acquisition is expected to start in fall 2022. Here, the status of the experiment and the first results of detector commissioning will be discussed.

The assessment of neutrino absolute mass scale is still a crucial challenge in today particle physics and cosmology. Beta or electron capture spectrum end-point study is currently the only experimental method which can provide a model-independent measurement of the absolute scale of neutrino mass. HOLMES is an experiment funded by the European Research Council to directly measure the neutrino mass. HOLMES will perform a calorimetric measurement of the energy released in the electron capture decay of the artificial isotope 163 Ho. In a calorimetric measurement, the energy released in the decay process is entirely contained into the detector, except for the fraction taken away by the neutrino. This approach eliminates both the issues related to the use of an external source and the systematic uncertainties arising from decays on excited final states. The most suitable detectors for this type of measurement are low-temperature thermal detectors, where all the energy released into an absorber is converted into a temperature increase that can be measured by a sensitive thermometer directly coupled with the absorber. This measurement was originally proposed by De Rujula and Lusignoli (Nucl Phys B 219:277, 1983 . https://doi.org/10.1016/0550-3213(83)90642-9 ), but only in the last decade the technological progress in detectors development has allowed to design a sensitive experiment. HOLMES plans to deploy a large array of low-temperature microcalorimeters with implanted 163 Ho nuclei. In this contribution we outline the HOLMES project with its physics reach and technical challenges, along with its status and perspectives.

This paper describes the production and chemical separation of the .sup.163 Ho isotope that will be used in several nuclear physics experiments aiming at measuring the neutrino mass as well as the neutron cross section of the .sup.163 Ho isotope. For this purpose, several batches of enriched .sup.162 Er have been irradiated at the Institut Laue-Langevin high flux reactor to finally produce 6 mg or 100 MBq of the desired .sup.163 Ho isotope. A portion of the Er/Ho mixture is then subjected to a sophisticated chemical separation involving ion exchange chromatography to isolate the Ho product from the Er target material. Before irradiation, a thorough analysis of the impurity content was performed and its implication on the produced nuclide inventory will be discussed.

The absolute neutrino mass is still an unknown parameter in the modern landscape of particle physics. The HOLMES experiment aims at exploiting the calorimetric approach to directly measure the neutrino mass through the kinematic measurement of the decay products of the weak process decay of 163 Ho. This low energy decaying isotope, in fact, undergoes electron capture emitting a neutrino and leaving the daughter atom, 163 Dy ∗ , in an atomic excited state. This, in turn, relaxes by emitting electrons and, to a considerably lesser extent, photons. The high-energy portion of the calorimetric spectrum of this decay is affected by the non-vanishing neutrino mass value. Given the small fraction of events falling within the region of interest, to achieve a high experimental sensitivity on the neutrino mass, it is important to have a high activity combined with a very small undetected pileup contribution. To achieve these targets, the final configuration of HOLMES foresees the deployment of a large number of 163 Ho ion-implanted TESs characterized by an ambitiously high activity of 300 Hz each. In this paper, we outline the status of the major tasks that will bring HOLMES to achieve a statistical sensitivity on the neutrino mass as low as 2 eV/c 2 .