Neutrinos are among the most enigmatic particles in the universe. They are omnipresent yet interact extremely rarely with matter. In cosmology, they influence the formation of large-scale galaxy structures, while in particle physics, their minuscule mass serves as an indicator of previously unknown physical processes. Precisely measuring the neutrino mass is therefore essential for a complete understanding of the fundamental laws of nature.
This is precisely where the KATRIN experiment with its international partners comes into play. KATRIN uses the beta decay of tritium, an unstable isotope of hydrogen, to measure the neutrino mass with the help of the energy distribution of the resulting electrons. Achieving this requires highly advanced technical components: the 70-meter-long beamline houses one an intense tritium source, as well as a high-resolution spectrometer with a diameter of 10 meters. This cutting-edge technology allows for unprecedented precision in direct neutrino mass measurements.
With the current data from the KATRIN experiment, an upper limit of 0.45 electron volts/c2 (which corresponds to 8 x 10- 37 kilograms) could be derived for the neutrino mass. Compared to the last results from 2022, the upper limit could thus be reduced by almost a factor of two.
Data Analysis
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 Prof. Kathrin Valerius from the Institut für Astroteilchenphysik at KIT, one of the two co-spokespersons of the experiment. Susanne Mertens from the Max-Planck-Institut für Kernphysik (MPIK) and Technical University Munich (TUM), adds: "With each campaign, we have gained new insights and further optimized the experimental conditions."
The evaluation of the complex data posed a challenge 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 from the Institut für Experimentelle Teilchenphysik (KIT), Co-Analysis Coordinator. Christoph Wiesinger (TUM/MPIK), also Co-Analysis Coordinator, adds: “We need to employ state-of-the-art analysis methods, with artificial intelligence playing a crucial role.”
Outlook on future measurements
"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,"- the KATRIN team lookd optimistically to the future. 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.
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 kiloelectronvolt/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.
The KATRIN Collaboration
Scientists from over 20 institutions across 7 countries are working on the KATRIN project.
The research group led by the newly appointed Director Susanne Mertens at the Max-Planck-Institut für Kernphysik (previously TU München) played a significant role in the results presented here. For instance, the researchers developed a new analysis method and, under the leadership of Co-Analysis Coordinator Christoph Wiesinger, made substantial contributions to the data analysis. The group was also involved in the necessary calibration measurements. The working group, which is now based at the MPIK, is also leading the TRISTAN detector development.
Original publikation
Direct neutrino-mass measurement based on 259 days of KATRIN data
KATRIN collaboration
Science 388 (issue 6743), 180–185 (2025). DOI: https://www.science.org/doi/10.1126/science.adq9592
Further references:
KATRIN Website: http://www.katrin.kit.edu
Science Perspective: https://www.science.org/doi/10.1126/science.adw9435
