![]() ![]() However, completely different strategies must be deployed to gather information on the absolute values, m i, of the neutrino mass states. Such measurements have determined that at least two of the three neutrino masses are larger than about 8 × 1 0 − 3 eV. The best tools for characterizing these mass differences have been experiments in which flavor oscillations are detected as neutrinos travel over large distances. These oscillations are only possible if neutrinos are massive, and the oscillation rate depends on the squared mass difference between the different neutrino mass states: Δ m i j 2 = m i 2 − m j 2, where i, j = 1, 2, 3. ![]() The smoking gun for neutrino mass is evidence of neutrino oscillations, in which one of the three flavors of neutrino transforms into another as it propagates. Since 1998, we have known that the neutrino is massive, in contradiction with the assumptions of the standard model of particle physics. Based on just one month’s worth of data, the Collaboration puts an upper limit of 1.1 eV on the neutrino mass, improving by a factor of 2 the mass limits derived by previous measurements that directly characterized the particle mass. After almost two decades of planning and preparation, the Karlsruhe Tritium Neutrino (KATRIN) Collaboration in Germany announces its first results. And since neutrinos are the most abundant massive particles in the Universe, the value of their mass would be important for cosmological models, influencing the formation of large-scale cosmic structures. Understanding what gives the neutrino a mass could help scientists pinpoint new physics beyond the standard model. Even its most basic property-its mass-is still unknown. The neutrino remains a truly mysterious particle, despite intense scientific efforts that have lasted for many decades. KATRIN Figure 1: Photograph of the electron spectrometer used by KATRIN. ![]()
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