Physicists Get Closer to Unveiling the Elusive Sterile Neutrino
Neutrinos, despite being incredibly elusive, are among the most abundant particles in the universe. The Standard Model of particle physics initially suggested three types of neutrinos. However, the discovery of neutrino oscillations has revealed that neutrinos possess mass and can transform into different types as they travel through space. This has sparked speculation about a fourth type, known as a sterile neutrino, which interacts even more weakly than the others. Confirming its existence would revolutionize our understanding of fundamental physics.
A groundbreaking study published in Nature presents the most precise direct search for sterile neutrinos to date. The research is a collaborative effort by the KATRIN collaboration, which analyzed the radioactive decay of tritium to detect subtle signs of this additional neutrino type.
The KATRIN (Karlsruhe Tritium Neutrino) experiment, located at the Karlsruhe Institute of Technology in Germany, is an impressive 70 meters long. It employs a powerful windowless gaseous tritium source, a high-resolution spectrometer for precise electron energy measurement, and a detector to record particles. Since its operation began in 2019, KATRIN has collected tritium β-decay data with unparalleled precision, specifically targeting the tiny deviations expected from a sterile neutrino.
What the Data Uncover About Sterile Neutrinos
In the recent Nature paper, the team reports the most sensitive tritium β-decay search for sterile neutrinos ever conducted. Between 2019 and 2021, KATRIN recorded approximately 36 million electrons over 259 days of data collection. These measurements were meticulously compared with detailed β-decay models, achieving an accuracy of better than one percent. The analysis revealed no evidence of a sterile neutrino.
This outcome eliminates a wide range of possibilities previously suggested by earlier anomalies, including unexpected deficits in reactor-neutrino experiments and gallium-source measurements, both of which hinted at a fourth neutrino. It also contradicts the Neutrino-4 experiment, which claimed evidence for such a particle.
KATRIN's exceptionally low background ensures that most detected electrons originate from tritium decay, enabling a highly accurate measurement of the energy spectrum. Unlike oscillation experiments, which observe neutrino identity changes over distance, KATRIN examines the energy distribution at the moment of neutrino creation. These methods, while probing different aspects of neutrino behavior, complement each other, providing strong evidence against the sterile neutrino hypothesis.
How KATRIN Enhances Other Experiments
Thierry Lasserre from the Max-Planck-Institut für Kernphysik in Heidelberg, who led the analysis, explains that KATRIN's new result is fully complementary to reactor experiments like STEREO. 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, these approaches consistently rule out light sterile neutrinos that would mix noticeably with known neutrino types.
Looking Ahead to More Data and New Detectors
KATRIN will continue collecting data until 2025, further enhancing its sensitivity and enabling even stricter tests for light sterile neutrinos. By the end of data collection in 2025, KATRIN will have recorded over 220 million electrons in the region of interest, increasing statistics by more than six times. This will allow the team to push the boundaries of precision and probe mixing angles below current limits.
An upgrade is planned for 2026 with the addition of the TRISTAN detector. TRISTAN will record the full tritium β-decay spectrum with unprecedented statistics. By bypassing the main spectrometer and directly measuring electron energies, 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, according to co-spokesperson Susanne Mertens from the Max-Planck-Institut für Kernphysik.
An International Scientific Endeavor
The KATRIN Collaboration brings together scientists from over 20 institutions across seven countries, showcasing the global effort behind one of the most precise neutrino experiments ever built.
