Breakthrough KATRIN Experiment Narrows Upper Limit of Neutrino Mass

Neutrinos—elusive fundamental particles generated in radioactive decay, solar activity, and cosmic events—are renowned for their extremely small mass. Now, new results suggest they may be even lighter than previously thought.

The Karlsruhe Tritium Neutrino (KATRIN) experiment, led by researchers at the Karlsruhe Institute of Technology in Germany, has established that the mass of the neutrino is less than 0.45 electronvolts (eV). This finding nearly halves the upper limit previously set by the same experiment. The results were recently published in the journal Science.

Neutrinos are the only fundamental particles whose mass remains unknown. Their mass is less than one-millionth that of an electron, and they were once believed to be massless. Understanding why neutrinos are so light is one of the most pressing questions in particle physics, and precisely measuring their mass is key to solving the puzzle.

KATRIN determines the mass of the electron antineutrino by studying the beta decay of tritium, a heavy isotope of hydrogen. In each decay event, a tritium nucleus emits an electron and an electron antineutrino. The neutrino’s mass limits the maximum energy of the emitted electron. By analyzing data from 36 million electrons, the KATRIN team was able to detect this minute effect with exceptional sensitivity.

The experiment will continue through the end of 2025. With additional unprocessed data, researchers hope to further narrow the range of possible neutrino masses. While cosmological observations have also placed limits on neutrino mass, those estimates depend on theoretical models of the early universe and are subject to uncertainties. In contrast, KATRIN’s results are model-independent, offering direct and robust evidence that advances our understanding of particle physics.