Physicists from the KATRIN collaboration have reported no evidence for a sterile neutrino in a precise analysis of tritium decay data. The findings, published in Nature, contradict earlier experimental claims and strengthen the case against a fourth neutrino type. The experiment, based in Germany, continues to gather more data for further tests.
Neutrinos, among the most abundant particles in the universe, are notoriously hard to detect. The Standard Model of particle physics recognizes three types, but anomalies in experiments have long suggested the possibility of a fourth, called a sterile neutrino, which would interact even more weakly and potentially upend our understanding of fundamental physics.
The Karlsruhe Tritium Neutrino (KATRIN) experiment, located at the Karlsruhe Institute of Technology in Germany, is designed to measure neutrino mass by examining the energies of electrons from tritium beta decay. Spanning over 70 meters, it features a tritium source, high-resolution spectrometer, and detector. Operational since 2019, KATRIN has now conducted the most sensitive direct search for sterile neutrinos to date.
Analyzing data from 259 days between 2019 and 2021, which included about 36 million electrons, the team found no distortions in the electron energy spectrum that would indicate a sterile neutrino. This result excludes a wide range of sterile neutrino possibilities hinted at by prior reactor and gallium experiments, and directly opposes claims from the Neutrino-4 experiment.
"Our new result is fully complementary to reactor experiments such as STEREO," said Thierry Lasserre of the Max-Planck-Institut für Kernphysik, 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."
KATRIN's low background ensures clean measurements at the point of neutrino creation, differing from oscillation studies that track changes over distance. The collaboration plans to collect data through 2025, aiming for over 220 million electrons to boost precision by a factor of six, according to co-spokesperson Kathrin Valerius of KIT.
An upgrade in 2026 will introduce the TRISTAN detector to probe heavier sterile neutrinos, possibly in the keV range linked to dark matter. "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," noted co-spokesperson Susanne Mertens of the Max-Planck-Institut für Kernphysik.
Involving over 20 institutions from seven countries, KATRIN exemplifies international scientific cooperation in particle physics.
