NIST Researchers Probe the Mass of the Electron Neutrino

Neutrinos are the lightest elementary particles in the universe. But exactly how much do they weigh? For three decades, physicists have sought answers to this question because the masses of the three known types of neutrinos – electron, tau, and muon – profoundly influenced the birth and structure of the cosmos and may help explain how elementary particles acquired their heft.

Studying neutrinos isn’t easy, however. The particles interact so weakly with their surroundings that instruments can only rarely record their direct presence. That’s why exquisitely sensitive detectors developed and built at the National Institute of Standards and Technology (NIST) played a critical role in a recent experiment examining the mass of the electron neutrino.

The experiment employed arrays of miniature sensors fabricated at NIST and known as transition edge sensors (TES). These devices, which act as highly sensitive thermometers, consist of thin superconducting films maintained at a temperature right at the tipping point between superconducting (current flowing with no resistance) and ordinary resistance. Within this narrow temperature range, each TES can detect an increase in temperature as small as a few millionths of a degree kelvin.

In the experiment, based in Italy and known as HOLMES, researchers embedded tiny amounts of the radioactive isotope holmium-163 into a gold film thermally coupled to each TES. About once a second, a holmium nucleus in each film captured one of its orbiting electrons. The capture is part of a radioactive decay in which holomium-163 transformed into dysprosium-163 along with an electron neutrino.

Every electron neutrino escaped the film, taking energy along with it. But all the energy associated with the newly minted dysprosium atoms, which resulted in a small rise in temperature, remained within the film and was measured by the TES arrays.

The smaller the amount of energy taken away by the electron neutrinos, the more available to raise the temperature of the dysprosium atoms. From Einstein’s famous equation equating energy with mass, the very smallest amount of energy that the electron neutrino can carry away is its mass.

Over a two-month period, the researchers used the TES arrays to measure the heat energy acquired by the dysprosium atoms during millions of radioactive decays. The team’s analysis revealed that the maximum possible mass of the electron neutrino is no greater than 27 electron volts (eV). By comparison, an electron has a mass of 511,000 eV.

Another experiment, the KATRIN neutrino experiment in Karlsruhe, Germany, has placed an even smaller upper bound on the mass of the electron neutrino. However, HOLMES is a pathfinder experiment; a larger version of HOLMES, operating for a longer time, could place an even tighter limit on the mass of the electron neutrino than KATRIN can.

The HOLMES team, which includes researchers from several institutions in Switzerland, France, and Italy, including the National Institute of Physics in Milan, the University of Milan-Bicocca, the National Laboratory of Gran Sasso in Assergi, the University of Genova, along with the University of Colorado in Boulder, reported the findings in the 135 issue of Physical Review Letters.

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Paper:

Alpert, B.K, Balata, M., Becker, D.T., Bennett, D.A., Borghesi, M., Campana, P., et al. Most stringent bound on electron neutrino mass obtained with a scalable low temperature microcalorimeter array. Phys. Rev. Lett. 135, 141801 – Published 29 September, 2025. DOI: https://doi.org/10.1103/s9vl-7n24