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). The German Research Foundation funded the work. The results were published in the journal "Physical Review Letters".
