Interior view of the KATRIN main spectrometer Credit: M. Zacher / KATRIN Coll.
Neutrinos, the mysterious and nearly massless particles that barely interact with anything, are revealing new secrets through the KATRIN experiment.
Using tritium decay and advanced spectrometry, KATRIN has slashed the upper limit on neutrino mass, pushing our understanding of fundamental physics into new territory. With 250 days of data already analyzed and more to come, researchers are optimistic about uncovering even more surprises. Future upgrades aim to detect hypothetical sterile neutrinos, potential dark matter candidates, and possibly revolutionize our view of the universe’s invisible side.
Neutrinos: The Universe’s Ghost Particles
Neutrinos are some of the most mysterious particles in the universe. They’re everywhere, streaming through space and even through our bodies, but they almost never interact with matter. In cosmology, they play a role in shaping the large-scale structure of the universe. In particle physics, their incredibly small mass hints at unknown processes beyond the current understanding of physics. Measuring that mass precisely is crucial to uncovering deeper laws of nature.
That’s where the KATRIN experiment comes in. An international collaboration, KATRIN is designed to directly measure the mass of neutrinos using a process called beta decay. Specifically, it studies the decay of tritium, a radioactive form of hydrogen. When tritium decays, it releases an electron and a neutrino. By examining the energy of the emitted electrons, scientists can infer the mass of the neutrinos with high precision.
To do this, KATRIN uses state-of-the-art technology. The experiment includes a 70-meter-long beamline containing a powerful tritium source and a massive, 10-meter-wide spectrometer that analyzes the electrons’ energy. This setup allows for the most sensitive direct measurements of neutrino mass ever achieved.
A New Benchmark in Neutrino Mass
Based on its current data, KATRIN has set a new upper limit for the neutrino mass: less than 0.45 electron volt/c2 (about 8 × 10-37 kilograms). That’s nearly twice as precise as the previous best result from 2022.
The quality of the first datasets has steadily improved since the start of measurements in 2019. “For this result, we have analyzed five measurement campaigns, totaling approximately 250 days of data collection from 2019 to 2021 – about a quarter of the total data expected from KATRIN,” explains Kathrin Valerius (KIT), one of the two co-spokespersons of the experiment.
Susanne Mertens (Max Planck Institute for Nuclear Physics (MPIK) and Technical University Munich (TUM)) adds: “With each campaign, we have gained new insights and further optimized the experimental conditions.”
High-Tech Analysis with AI Support
The evaluation of the extremely complex data posed an enormous challenge and required the highest level of precision from the international data analysis team. “The analysis of the KATRIN data is highly demanding, as an unprecedented level of accuracy is required,” emphasizes Alexey Lokhov (KIT), Co-Analysis Coordinator. Christoph Wiesinger (TUM/MPIK), Co-Analysis Coordinator, adds: “We need to employ state-of-the-art analysis methods, with artificial intelligence playing a crucial role.”
The KATRIN Collaboration, pictured at a meeting in October 2024. Credit: J. Wolf / KATRIN Coll.
Outlook for Future Measurements
The researchers look optimistically to the future: “Our measurements of the neutrino mass will continue until the end of 2025. Through the continuous improvement of the experiment and analysis, as well as a larger data set, we expect an even higher sensitivity.” – and possibly groundbreaking new discoveries,” says the KATRIN team.
KATRIN already leads the global field of direct neutrino mass measurements and has surpassed the results of previous experiments by a factor of four with its initial data. The latest findings indicate that neutrinos are at least a million times lighter than electrons, the lightest electrically charged elementary particles. Explaining this enormous mass difference remains a fundamental challenge for theoretical particle physics.
TRISTAN and KATRIN++: The Next Frontier
In addition to the precise measurement of the neutrino mass, KATRIN is already planning the next phase. Starting in 2026, a new detector system, TRISTAN, will be installed. This upgrade to the experiment will enable the search for sterile, a hypothetical particle, which interacts even more feebly than the known neutrinos. With a mass in the keV/c² range sterile neutrinos are a potential candidate for dark matter.
Additionally, KATRIN++ will launch a research and development program aimed at designing concepts for a next-generation experiment capable of achieving even more precise direct neutrino mass measurements.
Explore Further: Pinning Down a Ghost Particle: Neutrino Mass Measured With Unprecedented Precision
Reference: “Direct neutrino-mass measurement based on 259 days of KATRIN data” by KATRIN Collaboration, Max Aker, Dominic Batzler, Armen Beglarian, Jan Behrens, Justus Beisenkötter, Matteo Biassoni, Benedikt Bieringer, Yanina Biondi, Fabian Block, Steffen Bobien, Matthias Böttcher, Beate Bornschein, Lutz Bornschein, Tom S. Caldwell, Marco Carminati, Auttakit Chatrabhuti, Suren Chilingaryan, Byron A. Daniel, Karol Debowski, Martin Descher, Deseada Díaz Barrero, Peter J. Doe, Otokar Dragoun, Guido Drexlin, Frank Edzards, Klaus Eitel, Enrico Ellinger, Ralph Engel, Sanshiro Enomoto, Arne Felden, Caroline Fengler, Carlo Fiorini, Joseph A. Formaggio, Christian Forstner, Florian M. Fränkle, Kevin Gauda, Andrew S. Gavin, Woosik Gil, Ferenc Glück, Steffen Grohmann, Robin Grössle, Rainer Gumbsheimer, Nathanael Gutknecht, Volker Hannen, Leonard Hasselmann, Norman Haußmann, Klaus Helbing, Hanna Henke, Svenja Heyns, Stephanie Hickford, Roman Hiller, David Hillesheimer, Dominic Hinz, Thomas Höhn, Anton Huber, Alexander Jansen, Christian Karl, Jonas Kellerer, Khanchai Khosonthongkee, Matthias Kleifges, Manuel Klein, Joshua Kohpeiß, Christoph Köhler, Leonard Köllenberger, Andreas Kopmann, Neven Kovač, Alojz Kovalík, Holger Krause, Luisa La Cascio, Thierry Lasserre, Joscha Lauer, Thanh-Long Le, Ondřej Lebeda, Bjoern Lehnert, Gen Li, Alexey Lokhov, Moritz Machatschek, Martin Mark, Alexander Marsteller, Eric L. Martin, Christin Melzer, Susanne Mertens, Shailaja Mohanty, Jalal Mostafa, Klaus Müller, Andrea Nava, Holger Neumann, Simon Niemes, Anthony Onillon, Diana S. Parno, Maura Pavan, Udomsilp Pinsook, Alan W. P. Poon, Jose Manuel Lopez Poyato, Stefano Pozzi, Florian Priester, Jan Ráliš, Shivani Ramachandran, R. G. Hamish Robertson, Caroline Rodenbeck, Marco Röllig, Carsten Röttele, Milos Ryšavý, Rudolf Sack, Alejandro Saenz, Richard Salomon, Peter Schäfer, Magnus Schlösser, Klaus Schlösser, Lisa Schlüter, Sonja Schneidewind, Ulrich Schnurr, Michael Schrank, Jannis Schürmann, Ann-Kathrin Schütz, Alessandro Schwemmer, Adrian Schwenck, Michal Šefčík, Daniel Siegmann, Frank Simon, Felix Spanier, Daniela Spreng, Warintorn Sreethawong, Markus Steidl, Jaroslav Štorek, Xaver Stribl, Michael Sturm, Narumon Suwonjandee, Nicholas Tan Jerome, Helmut H. Telle, Larisa A. Thorne, Thomas Thümmler, Simon Tirolf, Nikita Titov, Igor Tkachev, Korbinian Urban, Kathrin Valerius, Drahoslav Vénos, Christian Weinheimer, Stefan Welte, Jürgen Wendel, Christoph Wiesinger, John F. Wilkerson, Joachim Wolf, Sascha Wüstling, Johanna Wydra, Weiran Xu, Sergey Zadorozhny and Genrich Zeller, 10 April 2025, Science.
The KATRIN Collaboration
The KATRIN Collaboration is an international research effort centered at the Karlsruhe Institute of Technology (KIT) in Germany, involving over 150 scientists from institutions around the world. Its goal is to precisely measure the mass of neutrinos, the lightest known particles in the Standard Model, using a model-independent, laboratory-based approach. KATRIN does this by analyzing the energy spectrum of electrons emitted during the beta decay of tritium. The experiment combines a high-intensity tritium source with one of the most sensitive spectrometers ever built, enabling unprecedented precision in neutrino mass measurements and pushing the frontiers of particle and astroparticle physics.
