KATRIN experiment rules out favored light sterile neutrino region

by Robert Schreiber
Berlin, Germany (SPX) Dec 08, 2025

Neutrinos are among the most abundant matter particles in the Universe, yet they interact so weakly that they are difficult to detect and study. The Standard Model includes three neutrino types, but their ability to oscillate between flavors shows they have mass and can change identity as they travel. For many years, anomalies in reactor and source experiments have pointed to the possible existence of a fourth, sterile neutrino that would not take part in standard weak interactions, raising the possibility of physics beyond the Standard Model.

In work now reported in Nature, the KATRIN collaboration has carried out a direct search for such sterile neutrinos by examining the beta decay of tritium. The Karlsruhe Tritium Neutrino (KATRIN) experiment was designed to determine the neutrino mass by precisely measuring the energy spectrum of electrons emitted in tritium beta decay. In this decay, the neutrino carries away part of the available energy, imprinting a subtle signature on the electron spectrum recorded by the experiment. If a sterile neutrino existed and mixed with the known neutrino states, it would sometimes be produced in the decay, creating a characteristic kink-like distortion in the electron energy distribution.

KATRIN is installed at the Karlsruhe Institute of Technology in Germany and extends over a length of about 70 meters. The setup consists of a high-luminosity windowless gaseous tritium source that emits electrons, a high-resolution electrostatic spectrometer system that determines their energy, and a detector that counts the transmitted electrons. Since 2019, KATRIN has recorded the tritium beta spectrum with high precision, searching for minute deviations from the expected shape, including the specific kink structure that would signal a sterile neutrino contribution.

In the new analysis, KATRIN provides the most sensitive sterile-neutrino search to date based on tritium beta decay. Between 2019 and 2021, the collaboration collected 36 million electrons over 259 live days and compared the observed spectrum with detailed beta-decay models, achieving sub-percent accuracy in the measurement. No evidence for a sterile neutrino emerged from these data, allowing the team to exclude a broad region of parameter space that had been favored by earlier anomalies in reactor-neutrino and gallium-source experiments, where small but statistically significant deficits in detected neutrinos had been interpreted as possible signs of an additional neutrino state. The KATRIN results also fully rule out the specific signal reported by the Neutrino-4 experiment. With a high signal-to-background ratio that ensures almost all recorded electrons originate from tritium beta decay, KATRIN obtains a clean view of the spectrum at the point of neutrino creation. Unlike oscillation experiments, which track how neutrino flavors change after propagation, KATRIN focuses on the initial energy distribution, making the two approaches complementary and together providing a stringent test that disfavors the light sterile-neutrino hypothesis.

"Our new result is fully complementary to reactor experiments such as STEREO," explains Thierry Lasserre (Max-Planck-Institut fur Kernphysik) in Heidelberg, who led the analysis. "While reactor experiments are most sensitive to sterile - active mass splittings below a few eV, KATRIN explores the range from a few to several hundred eV. Together, the two approaches now consistently rule out light sterile neutrinos that would noticeably mix with the known neutrino types."

Data collection at KATRIN will continue through 2025, which will further improve the sensitivity to light sterile neutrinos. "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 experiment is scheduled to receive a major upgrade with the TRISTAN detector, which will record the full tritium beta-decay spectrum with very high statistics while bypassing the main spectrometer and measuring electron energies directly. This configuration will extend the search to much higher sterile-neutrino masses in the kiloelectronvolt range. "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 fur Kernphysik).

Research Report:10.1038/s41586-025-09739-9