The APPEC Consortium convened its General Assembly for two in-person meetings in 2025: on the 4th and 5th of June in Prague, hosted by the Institute of Experimental and Applied Physics IEAP-CTU and on the 8th and 9th of December in Stockholm hosted by the Swedish Research Council.
These meetings provided an opportunity to review ongoing APPEC activities and collaborations in the European context and the adjacent scientific fields, and discuss the preparation of the upcoming strategic roadmap for astroparticle physics. Four General Assembly meetings have been scheduled for 2026. These sessions will provide important opportunities to review progress, discuss upcoming initiatives, and make collective decisions for the year ahead.
Discussions during the Potser Session at the APPEC Tech Forum 2025.
The IUVSTA Workshop 109 APPEC Tech Forum on Vacuum & Cryogenics took place in November in Maastricht, bringing together 103 participants from 11 countries. The program featured 17 academic presentations and 12 industry capability pitches, complemented by two panel discussions on cryogenics and vacuum technology. These sessions emphasized the importance of improving communication between scientists and industry engineers, particularly when involving industry in planning large-scale experiments.
The workshop concluded with a visit to the ET Pathfinder experiment at the University of Maastricht, a facility developing and testing technologies for the Einstein Telescope’s cryogenic interferometers.
The KATRIN experiment has searched with unprecedented precision for signs of a fourth type of neutrino, that could reveal new physics beyond the Standard Model. No signal was found, tightening the constraints on one of the most debated puzzles in neutrino research.
Inside the large electrostatic spectrometer, the heart of the Karlsruhe Tritium Neutrino Experiment KATRIN. Credits: Michael Zacher
Neutrinos, though nearly invisible, are among the most numerous matter particles in the Universe. The Standard Model recognizes three types, but the discovery of neutrino oscillations revealed they have mass and can change identity while propagating. For decades, puzzling experimental anomalies have suggested the presence of a fourth, sterile neutrino, one that interacts even more weakly. Finding it would transform our understanding of particle physics.
In a new study, published in Nature, the KATRIN collaboration presents the most precise direct search for sterile neutrinos through measurements of tritium β-decay.
The KATRIN (Karlsruhe Tritium Neutrino) experiment, built to determine the neutrino mass, measures the energy spectrum of electrons emitted in the β-decay of tritium. In this process, the energy carried away by the neutrino subtly shapes the detected electron spectrum. If an additional sterile neutrino existed, it would occasionally be emitted in the decay, producing a distinct distortion, or “kink”, in the electron energy spectrum. KATRIN, located at the Karlsruhe Institute of Technology in Germany, is a large experiment extending over 70 meters. It comprises three main components: a high-luminosity windowless gaseous tritium source that emits electrons, a high-resolution spectrometer system that measures their energy, and a detector that counts them. Since 2019, KATRIN has measured the tritium β-decay spectrum with unmatched precision, looking for small deviations, especially the characteristic kink expected
from a sterile neutrino.
The new Nature publication presents the most sensitive search to date for sterile neutrinos using the β-decay of tritium. KATRIN collected 36 million electrons over 259 days from 2019 to 2021 and compared them to a β-decay model, reaching sub-percent measurement accuracy. No sign of a sterile neutrino was found. The result excludes a large region of parameter space suggested by earlier anomalies: small but significant deficits observed in reactor-neutrino and gallium-source experiments that had hinted at a fourth neutrino state. It also fully rules out the Neutrino-4 experiment claim, which had reported evidence for such a signal. With an excellent signal-to-background ratio ensuring that almost all detected electrons come from tritium β-decay, KATRIN achieves a remarkably clean measurement of the spectral shape. In contrast to oscillation experiments, which study how neutrinos change flavor after traveling some distance, KATRIN probes the energy distribution at the point of creation. Relying on distinct detection methods, the two approaches complement each other and jointly deliver a powerful test that disfavors the sterile-neutrino hypothesis.
KATRIN’s new data (black) largely rule out the sterile-neutrino hints suggested by earlier reactor and gallium anomalies.
From DOI: 10.1038/s41586-025-09739-9
“Our new result is fully complementary to reactor experiments such as STEREO,” explains Thierry Lasserre (Max-Planck-Institut für Kernphysik) in Heidelberg, who led the analysis. “While reactor experiments are most sensitive to sterile–active mass splittings below a few eV², KATRIN explores the range from a few to several hundred eV². Together, the two approaches now consistently rule out light sterile neutrinos that would noticeably mix with the known neutrino types.”
With data collection continuing through 2025, KATRIN’s sensitivity will further increase, enabling even more stringent searches for light sterile neutrinos. “By the completion of data taking in 2025, KATRIN will have recorded more than 220 million electrons in the region of interest, increasing the statistics by over a factor of six,” says KATRIN co-spokesperson Kathrin Valerius (KIT). “This will allow us to push the boundaries of precision and probe mixing angles below the present limits.” In 2026, the KATRIN experiment will be upgraded with the TRISTAN detector, capable of recording the full tritium β-decay spectrum with unprecedented statistics. By bypassing the main spectrometer and measuring electron energies directly, TRISTAN will be able to explore much higher sterile-neutrino masses. “This next-generation setup will open a new window into the keV-mass range, where sterile neutrinos might even form the Universe’s dark matter,” says co-spokesperson Susanne Mertens (Max-Planck-Institut für Kernphysik).
The KATRIN Collaboration
Scientists from over 20 institutions across 7 countries are working on the KATRIN project.
On the 23rd and 24th of September, more than 100 researchers met in Zaragoza to discuss the new developments in Astroparticle Physics and in the neighboring fields that will shape the strategic recommendations of the next strategic roadmap.
The round table sessions at the APPEC Town Meeting provided additional input from the community. Currently an updated document is under preparation by the APPEC Scientific Advisory Committee.
KM3NeT has taken a major step forward with the creation of the KM3NeT Association Internationale Sans But Lucratif (AISBL), a new international non‑profit entity under Belgian law.
This structure, identified as the best legal framework through the EU‑supported KM3NeT‑Infradev2 project, will streamline and formalise all activities related to the construction, operation, scientific use, and future decommissioning of the infrastructure.
The AISBL was officially signed in Brussels on 18 June 2025 by representatives of the founding members—CNRS, INPP/NCSRD, the University of Valencia, INFN, and NWO‑I— and the Belgian Royal Decree stating the creation of the structure was published in October.
KM3NeT will start operation as an AISBL in January 2026.
Other KM3NeT institutes are expected to join soon as members or observers.
The main Hyper-Kamiokande cavern after excavation completed. Credits: Kamioka Observatory, ICRR (Institute for Cosmic Ray Research), The University of Tokyo
On July 31, 2025, the University of Tokyo completed excavation of the colossal cavern that will house the main detector volume of Hyper-Kamiokande, a next-generation, ultra-large water Cherenkov detector currently under construction in Hida City, Gifu, Japan.
Hyper-Kamiokande (Hyper-K) is a next-generation particle detector consisting of a gigantic water tank with a fiducial volume 8.4 times that of its predecessor, Super-Kamiokande, and is equipped with over 20,000 newly developed photodetectors. It is currently being constructed 600 meters underground beneath a mountain in Hida City, Gifu Prefecture, Japan, with the University of Tokyo leading the effort. In parallel, the High Energy Accelerator Research Organization (KEK) is leading the upgrade of the J-PARC neutrino beam and the construction of a new intermediate detector in Tokai Village, Ibaraki Prefecture. Through the combination of these efforts, the Hyper-Kamioande project aims to precisely measure neutrino properties and to search for proton decay, ultimately contributing to solving fundamental mysteries of the universe and testing the ideas of Grand Unified Theories. The Hyper-K project officially began in February 2020 with the allocation of its initial-year budget.
The Hyper-Kamiokande project is an international scientific research collaboration led by the University of Tokyo and the High Energy Accelerator Research Organization. As of July 2025, approximately 630 researchers from 22 countries are actively contributing to the project.
European institutes and companies are playing a large role in the construction and operation of the Hyper-Kamiokande experiment, with over 50% of the collaboration members from Europe leading the design, construction and installation of multiple elements of the various detectors of Hyper-Kamiokande.
Recovery of one of the last ANTARES detector line on May 2022. Credits: ANTARES
ANTARES, the first neutrino telescope in seawater operated in the Mediterranean for over 15 years before being decommissioned and dismantled in 2022. The ANTARES Legacy paper summarizes two decades of neutrino searches in the Mediterranean Sea, detailing the challenges, achievements, and scientific results. The paper presents the final analysis of cosmic diffuse neutrino flux based on 4541 days of data, reports constraints on steady neutrino sources in the Southern sky, and synthesizes multi-messenger searches, including neutrinos linked to TXS0506+056—the first discovered high-energy neutrino source.
Observatory’s view of the Virgo Cluster, offering a vivid glimpse of the variety in the cosmos. Visible are two prominent spiral galaxies, three merging galaxies, galaxy groups both near and distant, stars within our own Milky Way, and much more. Credit: NSF–DOE Vera C. Rubin Observatory
The NSF–DOE Vera C. Rubin Observatory, a major new scientific facility jointly funded by the U.S. National Science Foundation and the U.S. Department of Energy’s Office of Science, released its first imagery at an event in Washington, D.C. The imagery shows cosmic phenomena captured at an unprecedented scale. In just over 10 hours of test observations, NSF–DOE Rubin Observatory has already captured millions of galaxies and Milky Way stars and thousands of asteroids. The imagery is a small preview of Rubin Observatory’s upcoming 10-year scientific mission to explore and understand some of the Universe’s biggest mysteries.
The result of more than two decades of work, Rubin Observatory is perched at the summit of Cerro Pachón in Chile, where dry air and dark skies provide one of the world’s best observing locations. Rubin’s innovative 8.4-meter telescope has the largest digital camera ever built, which feeds a powerful data processing system. Later in 2025, Rubin will begin its primary mission, the Legacy Survey of Space and Time, in which it will ceaselessly scan the sky nightly for 10 years to precisely capture every visible change.
European physicists in astroparticle physics as well as in neighboring fields are planning the next generation of experiments to be built within the next decade. The success of the projects in direct dark matter detection, low energy neutrino physics, neutrino properties, gravitational wave detection as well as related accelerator-based experiments in particle and nuclear physics highly depends on challenging technologies in the domain of vacuum and cryogenics. The first event of this series, focusing on vacuum and cryogenics, took place in Darmstadt/Germany in 2012. A brochure was published after the meeting, providing an overview of participating experiments and companies.
For a thorough planning of all stakeholders it is important to carefully elaborate the timing of the projects and their needs as well as the market availability of key products. The APPEC Technology Forum is organized jointly with the international union of vacuum societies IUVSTA, national vacuum societies from the Netherlands and Germany, Maastricht University, NIKHEF and the Karlsruhe Institute of Technology (KIT) to identify synergies between projects from neighboring fields. It shall provide a discussion forum for companies, project scientist and funding agencies to define future ways of boosting cooperation to the benefit of all stakeholders.
This triennial meeting unites the particle, nuclear, and astroparticle physics communities, funding agencies, and major collaborations to explore synergies, share recent achievements, and address common challenges.
Topics include physics highlights, future projects, detector and computing advances, and updates on seven joint activities.
