FOR 2202: Neutrino Mass Determination by Electron Capture in Holmium-163 (ECHo)

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The determination of the absolute scale of the neutrino masses is one of the major challenges of today’s particle physics. Different approaches are fol-lowed to achieve a sensitivity on neutrino masses in the sub-eV range. Among them, experiments exploring the beta decay and electron capture processes of suitable nuclides can provide model-independent information on the electron (anti)- neutrino mass value purely using momentum and ener-gy conservation. The Electron Capture 163Ho experiment, ECHo, is aimed at investigating the electron neutrino mass in the sub-eV range by means of the analysis of the calorimetrically measured energy spectrum following the electron capture process of 163Ho. In the ECHo experiment, arrays of low tem-per-ature metallic magnetic calorimeters with high energy resolution and fast response time, having the 163Ho source embedded in the absorber, will be used to calorimetrically measure the EC spectrum. Independent measure-ments of the decay energy (QEC-value), for which different experiments cur-rently provide values ranging from 2.3 to 2.8 keV, will be performed using Penning-trap mass spectrometry. For the QEC measurements as well as for the calorimetric measurement of the 163Ho spectrum, high purity 163Ho sources will be produced. Detailed studies of the background and of meth-ods for its reduction will be carried out to increase the sensitivity of the calorimetric measurement. Based on the preparatory work and the presently on-going R&D in the research groups of ECHo we propose to establish a DFG Re-search Unit with the goal to achieve the required experimental conditions and the knowledge to be able to investigate the electron neutrino mass in the sub-eV range and to foster the collaboration between the groups involved. Within the scope of this Research Unit, we propose to develop and construct a medium-size experiment featuring an activity of 1 kBq (ECHo-1k) with the aim of investigating the value of the electron neutrino mass in the range be-low 10 eV/c2 by means of a high-precision, high-statistics calorimetric measure-ment of the 163Ho EC spectrum. The expected result will improve the limit on the electron neutrino mass by more than one order of magnitude with respect to the presently most stringent limit of m(νe) < 225 eV/c2 (95% CL).