Status of the HOLMES Experiment

Abstract

The assessment of the absolute \(\nu\) 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 \(\mu\)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 \(^{166m}\)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.