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The photo shows a detector module for the ECHo experiments developed and built at the Kirchhoff Institute for Physics. The detector chip is located in the middle; the four surrounding chips contain the Superconducting Quantum Interference Devices that read out the signals.
Photo Credit: © ECHo Collaboration
Scientific Frontline: Extended "At a Glance" Summary: The ECHo Experiment and Neutrino Mass
The Core Concept: The Electron Capture in Ho-163 (ECHo) experiment is a large-scale, international research collaboration dedicated to precisely determining the highly elusive mass of neutrinos through the analysis of radioactive decay.
Key Distinction/Mechanism: While similar studies approach their final sensitivity limits, ECHo isolates the energy released during the electron capture decay of the isotope Holmium-163. By utilizing metallic magnetic calorimeters operating at ultra-low temperatures (20 millikelvins), researchers can measure microscopic temperature fluctuations in the energy spectrum. These minute changes in atomic excitation energy allow scientists to deduce the mass of the ejected neutrino.
Origin/History: Spearheaded by spokesperson Prof. Dr. Loredana Gastaldo at Heidelberg University since 2011, the collaboration achieved a major milestone in March 2026. The team successfully adjusted the upper limit of the neutrino mass scale downward by approximately one order of magnitude compared to previous ECHo measurements, publishing their findings in Physical Review Letters.
Major Frameworks/Components:
- Holmium-163 (Ho-163) Decay: A radioactive process where a proton captures an electron, yielding a neutron and a neutrino, characterized by an exceptionally low energy release.
- Metallic Magnetic Calorimeters: Highly sensitive micro-detectors (approximately 200 micrometers in size) capable of registering fractional energy differences at near absolute zero.
- Energy Spectrum Analysis: Tracking slight variations in the energy distribution of atomic excitations to map the uncharged, "ghost-like" mass of neutrinos.
- Complementary Verification: Designed to complement and eventually surpass the sensitivity of the Karlsruhe Tritium Neutrino Experiment (KATRIN).
Branch of Science: Particle Physics, Experimental Physics, and Nuclear Physics.
Future Application: The successful optimization of these "cool" detectors paves the way for the ECHo-LE (Large Experiment) project, which is funded by an ERC Advanced Grant. This initiative will scale the operational detector array from 100 to 20,000 units to reach unprecedented levels of measurement sensitivity.
Why It Matters: Because neutrinos lack an electrical charge and interact very weakly with matter, their physical properties remain one of the greatest mysteries in modern science. Pinpointing their exact mass will enable physicists to formulate new theoretical frameworks beyond the standard model of particle physics, ultimately offering a deeper understanding of the fundamental evolution of the universe.
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| The figure shows the most precise electron capture spectrum of Holmium-163 to date, featuring 200 million events. Analyzing the end point range allowed for the downward adjustment of the upper limit for the neutrino mass by approximately one order of magnitude compared to previous ECHo measurements, and by a factor two compared to the result of the HOLMES Collaboration. Image Credit: © ECHo Collaboration |
New upper limit determined – Current research results are the basis for large-scale experiments to get closer to the mass of “ghost particles”
Their mass is extremely low, but how light are neutrinos really? A collaboration comprising German and international research groups has optimized its experiments to determine the mass of these “ghost particles”. In doing so, they succeeded in further adjusting downward the upper limit on the neutrino mass scale that had previously been determined in similar experiments. As part of the “Electron Capture in Ho-163 Experiment” (ECHo), the researchers are using the isotope Holmium-163 (Ho-163), whose decay processes allow for conclusions on the neutrino mass. According to ECHo spokesperson Prof. Dr Loredana Gastaldo, a scientist at Heidelberg University’s Kirchhoff Institute for Physics, the current results verify that even larger-scale investigations will be feasible in future to get even closer to the mass of neutrinos and ultimately precisely determine it.
Neutrinos are elementary particles with extremely low mass that have no electrical charge. Because their interaction with matter is very weak, the properties of these “ghost particles” are very difficult to determine. This is especially true for the neutrino mass, which has yet to be precisely measured, with only its upper limit being known. According to Loredana Gastaldo, determining the mass could pave the way for new theoretical models beyond the standard model of particle physics and thereby contribute to a better understanding of the evolution of our universe.
Several research groups worldwide are trying to determine the neutrino mass scale through the analysis of radioactive decays. The thus far smallest upper value has been obtained by the “Karlsruhe Tritium Neutrino Experiment” (KATRIN), which is, however, approaching its final sensitivity, as Prof. Gastaldo explains. The ECHo experiment has been designed to complement the KATRIN results and eventually reach an even better sensitivity. The collaboration, for which the scientist has served as spokesperson since 2011, includes research teams from Heidelberg, Mainz, Darmstadt, Tübingen, and Karlsruhe, as well as Geneva (Switzerland) and Grenoble (France).
As part of the ECHo experiments to determine the neutrino mass, the researchers are studying the energy released during the decay of Holmium-163. In this decay process, a proton in the atomic nucleus of this radioactive isotope captures an electron. The interaction between these two particles produces a neutron and a “ghost-like” neutrino, which is ejected with a specific energy. The mass of the neutrino causes a slight change in the energy distribution of the atomic excitations. “We can draw conclusions about the mass of the neutrino from the slight changes in the measured energy spectrum,” states Prof. Gastaldo. According to the experimental physicist, the isotope Holmium-163 is especially well suited for these measurements, because very little energy is released during its decay. That means that even tiny fluctuations in the spectral shape can be proven with appropriate detectors.
Metallic magnetic calorimeters are used for the ECHo experiments. These detectors were developed and built at the Kirchhoff Institute for Physics under the direction of Prof. Gastaldo. They are approximately 200 micrometers in size and operated at extremely low temperatures of 20 millikelvins, so that even the tiniest energy differences in the form of temperature fluctuations are evident. The Holmium-163 is embedded directly in the detectors at the RISIKO facility of Johannes Gutenberg University Mainz. Thanks to an improved detector design, approximately 200 million such Holmium-163 decay processes were observed for the first time during the latest experiment carried out at Heidelberg University.
This allowed the researchers to adjust downward the mass upper limit by approximately one order of magnitude compared to previous ECHo measurements – and by a factor two compared to the results of the HOLMES Collaboration, which also uses Holmium-163 to determine the neutrino mass. “This result reinforces the significance of the ECHo experiments and demonstrates that even larger-scale experiments using Holmium-163 will be possible in future,” stresses Loredana Gastaldo. To this end, she plans to increase the number of detectors from the current 100 to 20,000. For the “Electron Capture in Ho-163 – Large Experiment” (ECHo-LE) project, she has obtained an ERC Advanced Grant from the European Research Council (ERC).
Teams from Heidelberg University, the Max-Planck Institute for Nuclear Physics in Heidelberg, Johannes Gutenberg University 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 all contributed to the current research. Other contributors include researchers from the CERN European research center in Geneva (Switzerland) and the Institut Laue-Langevin in Grenoble (France).
Funding: The German Research Foundation funded the work.
Published in journal: Physical Review Letters
Title: Improved Limit on the Effective Electron Neutrino Mass with the ECHo-1k Experiment
Authors: F. Adam, Ahrens, L. E. Ardila Perez, M. Balzer, A. Barth, D. Behrend-Uriarte, S. Berndt, K. Blaum, F. W. H. Böhm, M. Braß, L. Calza, K. Chrysalidis, M. Door, H. Dorrer, Ch. E. Düllmann, K. Eberhardt, S. Eliseev, C. Enss, P. Filianin, A. Fleischmann, R. Gartmann, L. Gastaldo, M. Griedel, A. Göggelmann, R. Hammann, R. Hasse, M. W. Haverkort, S. Heinze, D. Hengstler, R. Jeske, J. Jochum, K. Johnston, N. Karcher, S. Kempf , T. Kieck, U. Köster, N. Kovac, N. Kneip, K. Kromer, F. Mantegazzini, B. A. Marsh, M. Merstorf, T. Muscheid, M. Neidig, Y. N. Novikov, R. Pandey, A. Reifenberger, D. Richter, A. Rischka, S. Rothe, O. Sander, R. X. Schüssler, S. Scholl, Ch. Schweiger, C. Velte, M. Weber, M. Wegner, K. Wendt, and T. Wickenhäuser (ECHo Collaboration)
Source/Credit: Universität Heidelberg
Reference Number: phy032526_02