Neutrinos are among the most elusive and enigmatic particles in the universe. These ghost-like particles pass through matter—including our own bodies—almost entirely undetected. Yet, despite their abundance, much about neutrinos remains unknown, especially their mass. Unlocking this mystery is critical to deepening our understanding of both cosmology and fundamental particle physics, as even a tiny mass could point to new physics beyond the Standard Model.

At the forefront of this quest is the KATRIN experiment (Karlsruhe Tritium Neutrino), an international collaboration designed to make the most sensitive direct measurement of the neutrino’s mass. By observing beta decay—specifically the decay of tritium, a radioactive isotope of hydrogen—KATRIN analyzes the energy of electrons emitted in the process. Since energy conservation links the electron’s energy to that of the neutrino, any deviation can offer a precise estimate of the neutrino’s mass.

To accomplish this, the experiment relies on a highly advanced 70-meter-long setup. At its core is a powerful tritium source and a massive, 10-meter-wide spectrometer that measures the energy of the decay electrons with unparalleled sensitivity. This configuration has enabled KATRIN to set a new upper limit on the neutrino mass: less than 0.45 electron volt per c², equivalent to around 8 × 10⁻³⁷ kilograms—nearly double the precision of its previous 2022 results.

Since the experiment began collecting data in 2019, the quality of the measurements has steadily improved. Analyzing five separate measurement campaigns over approximately 250 days of operation, researchers have significantly refined both the hardware and the analytical methods. Kathrin Valerius of the Karlsruhe Institute of Technology (KIT), one of the experiment’s co-spokespersons, notes that these efforts represent just a quarter of KATRIN’s total planned dataset. With more data and continued optimization, the team anticipates even tighter constraints on neutrino mass by the time the project concludes in 2025.

The complexity of KATRIN’s data posed a significant analytical challenge. Each measurement required extreme precision, prompting the team to incorporate artificial intelligence and other advanced computational tools into their analysis. Alexey Lokhov (KIT), Co-Analysis Coordinator, emphasized the unprecedented level of accuracy involved, while Christoph Wiesinger (TUM/MPIK) highlighted the essential role of AI in parsing the vast and subtle datasets. These tools have enabled KATRIN to outperform all previous direct neutrino mass experiments by a factor of four.

Current findings confirm that neutrinos are at least a million times lighter than electrons, which themselves are the lightest electrically charged particles. Understanding why this is the case remains one of the biggest open questions in physics, as the Standard Model does not fully explain such a disparity.

Looking ahead, the KATRIN collaboration is preparing for its next chapter. In 2026, a major upgrade will be introduced with TRISTAN, a new detector system designed to search for sterile neutrinos—hypothetical particles that would interact even more weakly than known neutrinos and could potentially account for dark matter. At the same time, the KATRIN++ research and development initiative will begin designing the next-generation experiment aimed at achieving even greater sensitivity.

With its groundbreaking methods and record-setting precision, KATRIN is not only redefining our understanding of neutrinos but also opening new pathways toward discovering the hidden architecture of the universe.

By Impact Lab