KATRIN Narrows Down the Range of Neutrinos’ Mass

The Science

Scientists have known about the existence of neutrinos - tiny fundamental particles that almost never interact with matter - for 95 years. Nobel Prize-winning work in 1998 showed that their mass is not zero. However, neutrinos' exact mass is still unknown. The international KArlsruhe TRItium Neutrino (KATRIN) experiment uses the process by which a specific isotope of hydrogen (tritium) breaks down to investigate the neutrino's mass. KATRIN measures tritium's beta-decay spectrum with unprecedented precision. This measurement allows it to search for the faint signature of the neutrino mass. Unlike similar experiments, KATRIN's direct method does not rely on any theoretical models of how the universe has evolved. Instead, it relies only on the fundamental fact in physics that energy is conserved. Based on KATRIN's measurements, scientists have determined that the mass of a neutrino is more than one million times lighter than an electron.

The Impact

The mass of a neutrino affects our understanding of nuclear and particle physics. In addition, it fundamentally shapes our understanding of the universe. Neutrinos were created in vast numbers in the first second after the Big Bang. Even though each individual neutrino is very light, their masses added up to shape the structure of the universe. Better understanding the neutrino mass provides insights into why the universe looks the way it does and the roles of other particles and forces.

Summary

By carefully analyzing 259 days' worth of data, the KATRIN collaboration found that the neutrino mass is less than 0.45 eV/c², or 8x10^-34 g. This is the best neutrino-mass information ever obtained from a direct, laboratory experiment. To achieve this result, the collaboration used a large source of tritium gas to supply 10¹¹ decays each second. A 10-meter-diameter spectrometer on the instrument uses electromagnetic fields to sort decay electrons according to their energies. This new result uses data with a novel running mode that reduces backgrounds by a factor of two. This improvement makes the data set more sensitive. Although KATRIN is located in Germany, U.S. scientists designed and built the primary detector system along with its data acquisition system. They have also made major contributions to the complex analysis.

KATRIN is still actively taking data. The new result represents only about a quarter of its expected final data set. After the next phase of data-taking is completed, scientists plan to upgrade KATRIN's beamline. It would search for a hypothesized new type of neutrino, which - if it exists - could be a type of dark matter.

Contact

Diana ParnoDepartment of Physics, Carnegie Mellon University [email protected]

Funding

This work was funded by the Helmholtz Association, the Ministry for Education and Research BMBF, the doctoral school KSETA at KIT, Helmholtz Initiative and Networking Fund, Max Planck Research Group, and Deutsche Forschungsgemeinschaft DFG in Germany; the Ministry of Education, Youth and Sport in the Czech Republic; Istituto Nazionale di Fisica Nucleare (INFN) in Italy; the National Science, Research and Innovation Fund via the Program Management Unit for Human Resources & Institutional Development, Research and Innovation in Thailand; and the Department of Energy Office of Science, Nuclear Physics, in the United States, as well as the European Research Council (ERC). Computing cluster support has been provided by the Institute for Astroparticle Physics at Karlsruhe Institute of Technology, Max Planck Computing and Data Facility (MPCDF), and the National Energy Research Scientific Computing Center (NERSC), a DOE Office of Science User Facility at Lawrence Berkeley National Laboratory.

Publications

Aker M., et al. (KATRIN collaboration), Direct neutrino-mass measurement based on 259 days of KATRIN data, Science, 388, 180-185 (2025), [DOI: 10.1126/science.adq9592]

Related Links

Precision Scale KATRIN Sets Record For Measuring Neutrino Mass, CMU press release

Neutrinos Are Shrinking, and That's a Good Thing for Physics, New York Times

The neutrino remains too light to be weighed-and that's oddly exciting, Science

Physicists narrow down neutrino's mysterious mass, Nature