Physicists Tighten the Net Around the Elusive Sterile Neutrino

High-precision measurements from the KATRIN experiment strongly limit the existence of light sterile neutrinos and narrow the search for new physics.

Neutrinos are extremely difficult to detect, yet they are some of the most abundant matter particles in the Universe. The Standard Model includes three known types, but discoveries showing that neutrinos oscillate revealed that they have mass and can change from one type to another as they move.

For many years, unexplained results from several experiments have raised the possibility of a fourth kind, known as a sterile neutrino, which would interact even more weakly than the others. Confirming its existence would fundamentally reshape modern particle physics.

In a new study published in Nature, the KATRIN collaboration reports the most sensitive direct search so far for sterile neutrinos, using detailed measurements of tritium β-decay.

The KATRIN experiment. From left to right, the rear wall and electron gun, the windowless gaseous tritium source, the transport and pumping section, the pre- and main spectrometers, and the focal-plane detector in which the electrons are detected and counted. Credit: KATRIN collaboration

How KATRIN searches for hidden neutrinos

The KATRIN (Karlsruhe Tritium Neutrino) experiment was designed to measure the mass of neutrinos by examining the energy spectrum of electrons released during the β-decay of tritium. In this decay process, part of the energy is carried away by the neutrino, subtly influencing the energy of the emitted electron.

If a sterile neutrino were produced in some of these decays, it would leave a recognizable feature, described as a “kink”, in the electron energy spectrum. The experiment is based at the Karlsruhe Institute of Technology in Germany and stretches over more than 70 meters.

Anticipated imprint of a 10 eV fourth neutrino mass state in the tritium β-decay spectrum. The associated kink emerges at E0−m4​, while the amplitude of the modification of the spectrum is set by the mixing strength sin⁡2(θee). The mixing effect is intentionally amplified here for visibility. Credit: KATRIN collaboration

It consists of three main elements: a high-intensity windowless gaseous tritium source, a high-resolution spectrometer to analyze electron energies, and a detector that records them. Since 2019, KATRIN has been collecting exceptionally precise measurements of the tritium β-decay spectrum, searching for even the smallest deviations that could signal the presence of a sterile neutrino.

The new Nature study represents the most sensitive tritium β-decay search for sterile neutrinos to date. Between 2019 and 2021, KATRIN recorded 36 million electrons over 259 days and compared the data with a detailed β-decay model, achieving measurement precision below one percent.

Transport of the KATRIN main spectrometer vessel, a major component of the experiment, to KIT’s Campus North site. Credit: KATRIN collaboration

The analysis found no evidence for a sterile neutrino. These results rule out a large portion of the parameter space suggested by earlier experimental anomalies, including deficits reported in reactor-neutrino and gallium-source studies that had pointed to a possible fourth neutrino. The findings also completely exclude the claim from the Neutrino-4 experiment, which had reported signs of such a particle.

Thanks to its very low background, almost all detected electrons can be confidently attributed to tritium β-decay, allowing KATRIN to measure the spectrum with exceptional clarity.

The KATRIN Collaboration in October 2024. Credit: KATRIN collaboration

Unlike oscillation experiments, which track how neutrinos change type after traveling some distance, KATRIN examines the energy distribution at the moment the neutrinos are produced. Together, these different approaches provide complementary evidence that strongly disfavors the existence of light sterile neutrinos.

“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.”

Pushing sensitivity toward darker candidates

With data collection continuing through 2025, KATRIN’s sensitivity will further increase, enabling even more stringent searches for light sterile neutrinos.

KATRIN’s new data (black) largely rule out the sterile-neutrino hints suggested by earlier reactor and gallium anomalies. Credit: KATRIN collaboration

“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).

Reference: “Sterile-neutrino search based on 259 days of KATRIN data” by The KATRIN Collaboration, 3 December 2025, Nature.

DOI: 10.1038/s41586-025-09739-9

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