According to Frederick Reines and Clyde L. Cowan Jr, “each new discovery of natural science broadens our knowledge and deepens our understanding of the physical universe; but at times these advances raise new and even more fundamental questions than those which they answer.1” Although referring to beta decay, they could have said this just as well about their discovery of the neutrino.

Credit: Michael Latz / DDP / AFP via Getty images

They gave the mass of the electron antineutrino to be less than “1/500 of the electron mass, if any”1 — with an emphasis on “if any”. Around 40 years later, the observation of neutrino oscillations by the Super-Kamiokande2 and the Sudbury Neutrino Observatory3,4 collaborations gave a conclusive answer to this question: neutrinos oscillate, and therefore they have mass.

In addition to lower bounds from oscillations, the mass of neutrinos can be determined by different methods. Cosmological probes, such as the temperature anisotropy of the cosmic microwave background or galaxy clustering, constrain the sum of the three mass values. In addition, limits can be extracted from the half-life in searches for neutrino-less double-beta decay, and one can also gain information about the neutrino mass from beta decay kinematics because the endpoint of the decay spectrum is sensitive to the presence of the neutrino.

In this issue of Nature Physics, the KATRIN Collaboration reports5 a direct measurement of the effective electron antineutrino mass from the beta decay of tritium, constraining it to below 0.9 eV c2 at 90% confidence level. Combined with their previous results6, the upper limit is further improved to 0.8 eV c2. In the accompanying News & Views, Angelo Nucciotti describes how challenging neutrino mass measurements are7.

Although the measurement by the KATRIN Collaboration advances our understanding of neutrinos, the words of Reines and Cowan still ring true: “the problem of detecting these cosmic end-products of all nuclear energy generation processes and the measurement of their characteristics presents a great challenge to the physics of to-day1.”