Neutrinos are extraordinarily difficult to detect, yet they are among the most abundant matter particles in the Universe. According to the Standard Model of particle physics, there are three known kinds. That picture changed when scientists discovered neutrino oscillations, a phenomenon showing that neutrinos have mass and can switch between types as they move through space. Over the years, several unexplained experimental results have fueled speculation about a fourth variety known as a sterile neutrino, which would interact even more weakly than the others. Confirming its existence would mark a major shift in our understanding of fundamental physics.

A new study published in Nature reports the most precise direct search so far for sterile neutrinos. The work comes from the KATRIN collaboration, which analyzed radioactive decays of tritium to look for subtle signs of an additional neutrino type.

The KATRIN (Karlsruhe Tritium Neutrino) experiment was originally designed to measure the mass of neutrinos. It does this by carefully tracking the energies of electrons released during the β-decay of tritium. When tritium decays, the neutrino carries away some energy, which slightly alters the energy pattern of the emitted electrons. If a sterile neutrino were sometimes produced instead, it would leave a recognizable distortion, or "kink," in that pattern.

Located at the Karlsruhe Institute of Technology in Germany, KATRIN stretches more than 70 meters in length. Its setup includes a powerful windowless gaseous tritium source, a high-resolution spectrometer that precisely measures electron energies, and a detector that records the particles. Since beginning operations in 2019, the experiment has collected tritium β-decay data with unmatched precision, specifically searching for the tiny deviations expected from a sterile neutrino.

What the Data Reveal About Sterile Neutrinos

In the new Nature paper, the team reports the most sensitive tritium β-decay search for sterile neutrinos to date. Between 2019 and 2021, KATRIN recorded about 36 million electrons over 259 days of data taking. These measurements were compared with detailed models of β-decay and achieved accuracy better than one percent. The analysis found no evidence of a sterile neutrino.

This result rules out a broad range of possibilities that had been suggested by earlier anomalies. Those anomalies included unexpected deficits seen in reactor-neutrino experiments and gallium-source measurements, both of which had hinted at a fourth neutrino. The findings also completely contradict the Neutrino-4 experiment, which had claimed evidence for such a particle.

KATRIN’s exceptionally low background means that nearly all detected electrons originate from tritium decay, allowing for a very clean measurement of the energy spectrum. Unlike oscillation experiments, which observe how neutrinos change identity after traveling some distance, KATRIN examines the energy distribution at the moment the neutrino is created. Because these methods probe different aspects of neutrino behavior, they complement each other and together provide strong evidence against the sterile neutrino hypothesis.

How KATRIN Complements Other Experiments

"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."

Looking Ahead to More Data and New Detectors

KATRIN will continue collecting data through 2025, which will further improve its sensitivity and allow even stricter tests for 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."

An upgrade is planned for 2026, when the TRISTAN detector will be added to the experiment. TRISTAN will record the full tritium β-decay spectrum with unprecedented statistics. By bypassing the main spectrometer and measuring electron energies directly TRISTAN will be able to investigate much heavier sterile neutrinos. "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).

An International Scientific Effort

The KATRIN Collaboration brings together scientists from more than 20 institutions across 7 countries, reflecting the global effort behind one of the most precise neutrino experiments ever built.