The spectrometer for the Karlsruhe Neutrino Experiment, or KATRIN.

Precision Scale KATRIN Sets Record For Measuring Neutrino Mass

By Joachim Hoffmann Email Joachim Hoffmann

Joachim Hoffmann
Heidi Opdyke

The international KArlsruhe TRItium Neutrino Experiment (KATRIN) at the Karlsruhe Institute of Technology (KIT), which includes researchers at Carnegie Mellon University, has 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 eV/c2) for the neutrino mass. With this result, KATRIN, which measures neutrino mass in the laboratory using a model-independent method, 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 yet nonzero 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 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 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.

We need to employ state-of-the-art analysis methods, with artificial intelligence playing a crucial role.

Christoph Wiesinger
TUM/MPIK

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," said Kathrin Valerius, one of the co-spokespersons of the experiment and deputy head of KIT.

Susanne Mertens, a director at the Max Planck Institute for Nuclear Physics (MPIK) and a faculty member at the Technical University of Munich (TUM) added: "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,” said Alexey Lokhov, a co-analysis coordinator for the experiment and a researcher at KIT.

Christoph Wiesinger (TUM/MPIK), co-analysis coordinator, added: “We need to employ state-of-the-art analysis methods, with artificial intelligence playing a crucial role.”

Diana Parno, Falco-DeBenedetti associate professor of physics at Carnegie Mellon, is the U.S. spokesperson for KATRIN and has been a member of the experiment since 2011. She joined Carnegie Mellon in 2017, and she and her group members provide analysis and simulation for the experiment.

“We’ve done a lot of work on understanding one of the background sources that needs to be accounted for in this analysis, improving one of the items in the systematic uncertainty budget,” Parno said.

Along with Parno, among the authors on the paper out today (April 11), are CMU doctoral students Byron Daniel and Gen Li.

Measurements of the neutrino mass will continue until the end of 2025, and Parno and others said they anticipate narrowing the allowed range of the neutrino mass even further.

“We’re working hard to analyze and understand additional data that has been collected since the paper was written,” Parno said.

KATRIN 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 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 neutrinos in the keV/c2 mass range, 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.