Neutrinos are at Least a Million Times Lighter Than Electrons

Precision scale KATRIN at the Karlsruhe Institute of Technology sets a new upper limit for the mass of neutrinos.

The international KArlsruhe TRItium Neutrino Experiment (KATRIN) at the Karlsruhe Institute of Technology (KIT) has once again surpassed its own achievements. The latest data, recently published in Science, establish an upper limit of 8 x 10^-37 kg (or in scientific language 0.45 electronvolt/c²) for the neutrino mass. With this result, KATRIN, which measures neutrino mass in the laboratory using a model-independent method, has once again set a world record.

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, which also includes the Johannes Gutenberg University Mainz (JGU), comes into play. KATRIN utilizes the beta decay of tritium, an unstable hydrogen isotope, to assess the mass of neutrinos. The energy distribution of the electrons resulting from the decay enables a direct kinematic determination of the neutrino mass. 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.

Evaluation of the Data

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."

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."

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.

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 kilo electronvolt/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 contact person for the KATRIN collaboration at JGU is Dr. Larisa Thorne. She is a postdoc in the working group of Prof. Dr. Martin Fertl and part of the PRISMA⁺ Cluster of Excellence. She has been working on KATRIN since 2017. Her contributions to this new result included analyzing the first two datasets, characterizing plasma systematics, taking shifts to gather new data, and maintaining critical detector hardware.