HOLMES - The Electron Capture Decay of Ho-163 to Measure the Electron Neutrino Mass with sub-eV sensitivity

HOLMES is an experiment aimed at directly measuring the electron neutrino mass using the electron capture (EC) decay of ¹⁶³Ho.

¹⁶³Ho + e^- → ¹⁶³Dy^* + ν_e

A measurement fixing the value of the absolute neutrino mass represents a major breakthrough with profound consequences in particle physics and cosmology. In fact, due to their abundance as big-bang relics, massive neutrinos strongly affect the large-scale structure and dynamics of the universe. In addition, the knowledge of the scale of neutrino masses, together with their hierarchy pattern, is invaluable to clarify the origin of fermion masses beyond the Higgs mechanism.
The innovation of HOLMES lies in the calorimetric measurement of the energy released in the decay of ¹⁶³Ho. This allows to measure all the atomic de-excitation energy, except the fraction carried away by the neutrino. A finite neutrino mass m_ν causes a deformation of the energy spectrum which is truncated at Q-m_ν, where Q is the EC transition energy. The sensitivity depends on Q - the lower the Q, the higher the sensitivity - and ¹⁶³Ho is an ideal isotope with the Q at 2.83 keV. The direct measurement exploits only energy and momentum conservation and is therefore completely model-independent. At the same time, the calorimetric measurement eliminates systematic uncertainties arising from the use of external beta sources, as in experiments with beta spectrometers, and minimizes the effect of the atomic de-excitation process uncertainties.

 HOLMES will deploy a large array of low temperature microcalorimeters with implanted ¹⁶³Ho nuclei. The resulting mass sensitivity will be as low as few electronvolts. 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 0.1eV.
The cutting edge detection techniques developed for HOLMES will has an impact in many frontier fields like astrophysics, material analysis, nuclear safety and diagnostic, archaeometry, and quantum communication.