Abstract
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 ^{163}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 ^{163}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.
Highlights
The value of the neutrino mass has many implications, from cosmology to the Standard Model of particle physics
Thirty years ago it was suggested that electron capture (EC) decays with low Q-values could be used as an alternative to single beta decay for the direct determination of the electron neutrino mass
Background in an experiment like HOLMES is caused by cosmic rays – both muons crossing the microcalorimeter absorber and electromagnetic showers caused by nearby interactions of muons, by environmental radiation – such as environmental γ s or X-rays and Auger electrons from surrounding materials, and by radioactive contaminations inside the microcalorimeter absorbers
Summary
The value of the neutrino mass has many implications, from cosmology to the Standard Model of particle physics. An alternative to spectrometry is provided by calorimetric measurements in which the beta source is embedded in the detector In this configuration the energy emitted in the decay is entirely measured by the detector, except for the fraction taken away by the neutrino. This approach eliminates both the problematics of an external source and the systematic uncertainties coming from the excited final states. Several investigations have been carried out on microcalorimeters for a calorimetric neutrino mass measurement with 187Re. About 10 years ago, two small scale pilot experiments were carried out with thermal detectors containing 187Re: the MANU [17,18] and MIBETA [19,20] experiments. After several years of attempts metallic rhenium seems to be not fully compatible with the technical requirements of MARE and the focus of the community is shifting from this isotope to 163Ho [22]
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