GSI Press release, 12th February 2026 The world’s largest neutrino detector has been successfully upgraded The name “IceCube” not only serves as the title of the experiment, but also describes its appearance. Embedded in the transparent ice of the South Pole, a three-dimensional grid of more than 5,000 extremely sensitive light sensors forms a giant cube with a volume of one cubic kilometer. This unique arrangement serves as an observatory for detecting neutrinos, the most difficult elementary particles to detect. In order to detect neutrinos, they must interact with matter, creating charged particles whose light can be measured. These light measurements can be used to determine information about the properties of neutrinos. However, the probability of neutrinos interacting with matter is extremely low, so they usually pass through it without leaving a trace, which makes their detection considerably more difficult. For this reason, a large detector volume is required to increase the probability of interaction, and state-of-the-art technology is crucial for detecting such rare interactions. The basic operating principle of IceCube is to detect the light that is produced when a neutrino interacts with the ice. IceCube acts like a telescope that “sees” neutrinos. This characteristic blue Cherenkov light travels through the ice and is detected by sensors called digital optical modules (DOMs). Using these measurements, researchers can then reconstruct the energy and direction of the original neutrino. Since 2010, the IceCube Neutrino Observatory has been searching for high-energy neutrinos from space. In recent years, it has already provided important insights into the nature of these particles and the sources of these high-energy neutrinos in the universe. For example, it offered a first glimpse into the interior of an active galaxy. The recently completed upgrade of the observatory will ensure that the experiment will provide even more information about the properties of neutrinos and the cosmos. Scientists from the working group of Professor Dr. Sebastian Böser from the Institute of Physics and the PRISMA++ Cluster of Excellence at Johannes Gutenberg University Mainz (JGU) are part of the IceCube Collaboration. The collaboration has been represented at JGU since 1999, initially under the leadership of Professor Dr. Lutz Köpke. “In Mainz, we are primarily researching neutrinos at the lower end of the energy spectrum detectable by IceCube, such as those produced in the atmosphere or in supernova explosions. These are difficult to detect, but they can also provide us with new insights into the properties of neutrinos themselves,” explains Böser. More sensors improve the telescope The main array of IceCube consists of 86 sensor strings embedded in the ice at intervals of 125 meters. As part of the IceCube upgrade, six new strings were installed between December 2025 and January 2026. This added over 650 modern photodetectors and calibration devices to the existing IceCube detector. The new instruments will improve our understanding of how light emitted by neutrino interactions in the ice propagates through the detector. Thanks to the higher instrument density, the experiment can now measure signals at lower energies that were previously unattainable. This increases the “sharpness” of the telescope, making it more sensitive to the properties of neutrinos. In addition, the higher resolution achieved by the upgrade can also be applied retroactively to data already collected and stored during the first ten years of IceCube operation, resulting in an immediate and significant improvement. An innovative type of module The new components of the upgrade also include nine wavelength-shifting optical modules (WOMs): innovative detectors specialized for UV light. “With IceCube, we want to measure Cherenkov light. This light has a large UV component that the DOMs cannot measure. This means that a large part of the light produced during neutrino interactions is lost because the sensors are not sensitive enough for it,” explains Lea Schlickmann, a PhD student in Böser’s group and the person primarily responsible for building these modules. “The WOMs have a tube coated with a special wavelength-shifting paint. When UV photons hit this tube, their wavelength is shifted into the visible range and they are then directed to the so-called photomultipliers, where they are detected.” The WOMs were developed, produced, and tested in Mainz in collaboration with research groups from Wuppertal and Madison, with additional support from Uppsala and Berlin. These first modules serve as proof of principle for their performance and their measurements of UV Cherenkov light in ice. “In the future, WOMs will be able to provide extremely important information about neutrinos and their origin in the universe. They would be particularly suitable for detecting neutrinos produced in a supernova, which would be extremely interesting to observe,” says Schlickmann. In addition to her contribution to the hardware development of the detector, Schlickmann was also part of the first group of researchers allowed to travel to the South Pole to work on the IceCube upgrade. There, she not only tested the WOMs one last time before they were installed in the ice, but also helped with all kinds of tasks necessary for the success of the mission – from shoveling snow to clear equipment to testing and loading the first 300 modules. The IceCube collaboration consists of over 450 physicists from 58 institutions in 14 countries. This international team is leading the scientific program, and many of its members contributed to designing and constructing the detector. The IceCube Neutrino Observatory is mainly funded by the National Science Foundation (NSF) in the United States, with significant support from partner organizations worldwide. Germany is the second-largest contributor, with eleven institutions, and makes a significant and visible contribution to IceCube through funding from the Federal Ministry of Research, Technology, and Space (BMFTR). In addition to JGU, the collaboration includes Friedrich-Alexander University Erlangen-Nuremberg, Humboldt University Berlin, Karlsruhe Institute of Technology, Ruhr University Bochum, RWTH Aachen University, Technical University Dortmund, Technical University Munich, University of Münster, University of Wuppertal, and the German Electron Synchrotron (DESY).