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

European Research Council (ERC)  Advanced Grant n. 340321 - HOLMES

Principal Investigator: Prof. S. Ragazzi

Host Institution: INFN

Additional beneficiary: Università di Milano-Bicocca (Scientific Coordinator A. Nucciotti)

Experiment start:  February 1st, 2014

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

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 mν causes a deformation of the energy spectrum which is truncated at Q-mν, 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 a Q around 2.5keV. 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 0.4eV. 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 also have an impact in many frontier fields like astrophysics, material analysis, nuclear safety and diagnostic, archaeometry, and quantum communication.

HOLMES goals

  • neutrino mass measurement with a statistical sensitivity as low as 0.4 eV
  • proof potential and scalability of the approach
    • precise determination of transition Q value
    • systematic errors assessment

HOLMES baseline

  • TES detectors with Au absorber implanted with 163Ho
    • 6.5x1013 nuclei in each detector → 300 dec/sec
    • energy resolution ΔE≈1eV and resolving time τR≈1μs
  • 1000 channel multiplexed array
    • 6.5x1016  163Ho nuclei → ≈18μg
    • 3x1013 events in 3 years measuring time

HOLMES collaboration

Statistical sensitivity achievable by HOLMES baseline configuration
Poster presented at Neutrino 2014 conference,
June 1-7, 2014 - Boston, Ma, USA

  Slides presented at the INFN-CSN2 meeting in Povo (TN) on September 2013

  Slides of LNGS seminar, May 2014

  Slides of PSI seminar, May 2017

Diploma thesis, PhD thesis and Post-doc grants are available on many HOLMES topics.


  1. A. de Rújula and M. Lusignoli, "Calorimetric measurements of 163-Holmium decay as tools to determine the electron neutrino mass", Phys. Lett. B., vol. 118, issue 4-9, pp. 429–434 (1982)  DOI;
  2. M. Galeazzi et al., "The Electron Capture Decay of 163-Ho to Measure the Electron Neutrino Mass with sub-eV Accuracy", accepted for publication on Phys. Rev. D, arXiv:1202.4763
  3. G. J. Barker et al., "The Future of Neutrino Mass Measurements: Terrestrial, Astrophysical, and Cosmological Measurements in the Next Decade. Highlights of the NuMass 2013 Workshop. Milano, Italy, February 4 - 7, 2013",  arXiv:1309.7810
  4. A. Nucciotti, "HOLMES - The Electron Capture Decay of 163Ho to Measure the Electron Neutrino Mass with sub-eV sensitivity", seminar held at LNGS, May 2014
  5. A. Nucciotti, "Statistical sensitivity of 163-Ho electron capture neutrino mass experiments",  Eur. Phys. J. C 74, 3161 (2014). arXiv:1405.5060
  6. B. Alpert et al., "HOLMES - The Electron Capture Decay of 163Ho to Measure the Electron Neutrino Mass with sub-eV sensitivity", Eur. Phys. J. C 75, 112 (2015). arXiv:1412.5060
  7. A. Nucciotti et al., "The Use of Low Temperature Detectors for Direct Measurements of the Mass of the Electron Neutrino", Advances in High Energy Physics, 2016, 915304. arXiv:1511.00968

HOLMES project is funded by ERC Advanced Grant n. 340321 - HOLMES