Abstract
The determination of the absolute scale of the neutrino masses is one of the most challenging present questions in particle physics. The most stringent limit, $m(\bar{\nu}_{\mathrm{e}})<2$eV, was achieved for the electron anti-neutrino mass \cite{numass}. Different approaches are followed to achieve a sensitivity on neutrino masses in the sub-eV range. Among them, experiments exploring the beta decay or electron capture of suitable nuclides can provide information on the electron neutrino mass value. We present the Electron Capture $^{163}$Ho experiment ECHo, which aims to investigate the electron neutrino mass in the sub-eV range by means of the analysis of the calorimetrically measured energy spectrum following electron capture of $^{163}$Ho. A high precision and high statistics spectrum will be measured with arrays of metallic magnetic calorimeters. We discuss some of the essential aspects of ECHo to reach the proposed sensitivity: detector optimization and performance, multiplexed readout, $^{163}$Ho source production and purification, as well as a precise theoretical and experimental parameterization of the calorimetric EC spectrum including in particular the value of $Q_{\mathrm{EC}}$. We present preliminary results obtained with a first prototype of single channel detectors as well as a first 64-pixel chip with integrated micro-wave SQUID multiplexer, which will already allow to investigate $m(\nu_{\mathrm{e}})$ in the eV range.
Highlights
The current Standard Model (SM) is a very successful and accurate description of particle physics, we know it is incomplete and an extension is needed to understand the structure of the universe, the hierarchy of masses and to achieve a grand unification
Each event is detected by a device with a well-defined response, which is characterized by a finite time resolution and an instrumental response function, typically a Gaussian characterized by its full-width at half maximum (FWHM)
To achieve very good noise performance as well as to ensure that the system bandwidth is large enough to retain the fast signal rise time of metallic magnetic calorimeters (MMCs), both properties that are essential for the ECHo experiment, two dc-SQUIDs are very often arranged in a two-stage SQUID setup [69]
Summary
The current Standard Model (SM) is a very successful and accurate description of particle physics, we know it is incomplete and an extension is needed to understand the structure of the universe, the hierarchy of masses and to achieve a grand unification. Direct kinematical methods allow to perform a model-independent measurement of the electron neutrino and antineutrino mass with high sensitivity and low systematic uncertainties. In the past this approach was mainly driven by the investigation of the 3H beta spectrum. KATRIN, the KArlsruhe TRItium Neutrino experiment [10], is designed to either measure the electron-antineutrino mass down to 0.3 eV/c2 or alternatively give an upper limit of. Together with KATRIN, two other experiments are designed to improve the limit on the electron anti-neutrino mass analyzing the 3H beta-spectrum, these are Project8 [14] and PTOLEMY [15]. The Electron Capture in 163Ho experiment, ECHo, is designed to investigate the electron neutrino mass in the sub-eV region by the analysis of the calorimetrically measured electron capture spectrum of 163Ho
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