Measuring the electron neutrino mass with improved sensitivity: the HOLMES experiment

HOLMES is a new experiment aiming at directly measuring the neutrino mass with a sensitivity below 2 eV . HOLMES will perform a calorimetric measurement of the energy released in the decay of ¹⁶³Ho. The calorimetric measurement eliminates systematic uncertainties arising from the use of external beta sources, as in experiments with spectrometers. This measurement was proposed in 1982 by A. De Rujula and M. Lusignoli, but only recently the detector technological progress has allowed to design a sensitive experiment. HOLMES will deploy a 1000 pixels array of low temperature microcalorimeters with implanted ¹⁶³Ho nuclei. HOLMES, besides being an important step forward in the direct neutrino mass measurement with a calorimetric approach, will also establish the potential of this approach to extend the sensitivity down to 0.1 eV and lower. The detectors used for the HOLMES experiment will be Mo/Cu bilayers TESs (Transition Edge Sensors) on SiN^x membrane with gold absorbers. Microwave multiplexed rf-SQUIDs are the best available technique to read out large array of such detectors. An extensive R&D activity is in progress in order to maximize the multiplexing factor while preserving the performances of the individual detectors. To embed the ¹⁶³Ho into the gold absorbers a custom mass separator ion implanter is being developed. The current activities are focused on the the single detector performances optimization and on the ¹⁶³Ho isotope production and embedding. A preliminary measurement of a sub-array of 4× 16 detectors is planned late in 2017. In this contribution we present the HOLMES project with its technical challenges, its status and perspectives.