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Their mass is extremely small, but just how light are neutrinos really? An international collaboration has now optimised its experiment to determine the mass of these elementary particles, also titled as ‘ghost particles’. In doing so, the scientists have succeeded in further lowering the upper limit for the neutrino mass previously determined in similar measurements. As part of the “Electron Capture in Ho-163 Experiment” (ECHo), the scientists are using the isotope holmium-163 (Ho-163), which allows conclusions to be drawn about the neutrino mass during its decay processes.
Neutrinos are elementary particles with an extremely low mass and no electric charge. As they interact only very weakly with matter, it is extremely difficult to determine their properties. This applies in particular to the neutrino mass, which has not yet been measured precisely and is known only as an upper limit. However, its precise determination could pave the way for new theoretical models beyond the Standard Model of particle physics – and thus contribute to a better understanding of the evolution of our universe.
Several research groups around the world are working to determine the neutrino mass by analysing radioactive decays. The lowest upper limit to date was measured as part of the ‘Karlsruhe Tritium Neutrino Experiment’ (KATRIN), which investigates the radioactive decay of tritium. “ECHo was designed to complement the results from the KATRIN project and to achieve even greater sensitivity in the future,” explains Prof. Dr Loredana Gastaldo, a researcher at the Kirchhoff Institute for Physics at Heidelberg University. The ECHo collaboration, of which she has been the spokesperson since 2011, comprises research teams from Heidelberg, Mainz, Darmstadt, Tübingen and Karlsruhe, as well as from Geneva (Switzerland) and Grenoble (France).
To determine the neutrino mass, the researchers are investigating the energy released during the decay of holmium-163. During this decay process, a proton in the nucleus of the radioactive isotope ‘captures’ an electron. The interaction between these two particles produces a neutron and a neutrino. Whilst the neutrino cannot be detected directly, its mass causes an extremely small change in the energy distribution of the measured atomic excitations. “From the tiny changes in the measured energy spectrum, we can draw conclusions about the neutrino mass,” explains Prof. Gastaldo. The isotope holmium-163 is particularly well suited to these measurements because very little energy is released during its decay. This means that even very small fluctuations in the spectral shape can be detected using suitable detectors.
The ECHo experiments utilise metallic magnetic calorimeters developed and manufactured at the Kirchhoff Institute for Physics at Heidelberg University. They are only about 200 micrometres in size and are operated at extremely low temperatures of 20 thousandths of a Kelvin, so that even the smallest energy differences become apparent in the form of temperature fluctuations. Thanks to an improved detector design, the experiment now carried out has, for the first time, enabled the observation of around 200 million such holmium-163 decay processes. In this process, the radioactive holmium-163 was embedded directly into the detectors in order to reduce systematic errors associated with measurements using separate radioactive sources and detectors.
“This method increases the sensitivity of the measurements; however, the gold layer used to embed the holmium in the detectors could influence the measurement result,” explains Prof. Dr Klaus Blaum of the Max-Planck-Institut für Kernphysik (MPIK). “For this reason, our group at MPIK carried out a comparative measurement for the endpoint energy, derived from the mass difference between holmium-163 and dysprosium-163, in an ion trap and found no significant deviation in the ECHo measurement.”
The scientists were thus able to revise the upper limit of the neutrino mass downwards by about an order of magnitude compared with earlier ECHo measurements – and by a factor of two compared with the result from the HOLMES collaboration, which also uses holmium-163 to determine the neutrino mass and has achieved the most accurate result to date using this process. “This result underscores the importance of the ECHo experiments and demonstrates that even larger-scale investigations using holmium-163 will be possible in the future,” emphasises Gastaldo. To this end, the collaboration plans to expand the number of detectors from the current 100 to 20,000. For this project, “Electron Capture in Ho-163 – Large Experiment” (ECHo-LE), Gastaldo has received funding from the European Research Council (ERC) in the form of an ERC Advanced Grant.
Teams from Heidelberg University, the Max-Planck-Institut für Kernphysik in Heidelberg, the University of Mainz, the Helmholtz Institute Mainz, the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt, the University of Tübingen and the Karlsruhe Institute of Technology have contributed to the current research. Researchers from the European research centre CERN in Geneva (Switzerland) and the Institut Laue-Langevin in Grenoble (France) were also involved. The work was funded by the German Research Foundation (DFG). The research findings have been published in the journal “Physical Review Letters”.
Original publication:
F. Adam, et al.: Improved limit on the effective electron neutrino mass with the ECHo-1k experiment. Physical Review Letters (March 2026), https://doi.org/10.1103/lqkb-hylx
Further informationen:
ECHo-collaboration – www.kip.uni-heidelberg.de/echo
