Physicists with the KArlsruhe TRItium Neutrino (KATRIN) experiment report the most precise measurement of the upper mass limit of the neutrino to date, establishing it as 0.45 electron volts (eV) — less than one-millionth the mass of an electron.
Neutrinos are the most abundant particles in the Universe and exist as three distinct types or flavors: electron neutrino, muon neutrino, and tau neutrino.
These flavors oscillate, meaning a single neutron can transform into each type as it travels, providing compelling evidence that neutrinos possess mass that contradicts the Standard Model’s original assumption of massless neutrinos.
However, their exact mass remains one of the great mysteries of particle physics.
In a new paper in the journal Science, physicists with the KATRIN Collaboration present the results of the first five measurement campaigns of the KATRIN experiment.
“The KATRIN experiment determines the neutrino’s mass by analyzing the beta decay of tritium,” they explained.
“During this decay, a neutron transforms into a proton, emitting both an electron and an electron antineutrino — the latter being the neutrino’s antiparticle.”
“By analyzing the distribution of total decay energy between the emitted electron and the electron antineutrino, the neutrino’s mass can be inferred.”
Over 259 days between 2019 and 2021, the KATRIN physicists measured the energy of approximately 36 million electrons — a dataset six times larger than previous runs.
The findings establish the most stringent laboratory-based upper limit on the effective electron neutrino mass, placing it at < 0.45 eV with a 90% confidence level.
This result marks the third refinement of the neutrino mass limit and improves upon the previous limit by a factor of 2.
“For this result we have analyzed five measurement campaigns, totaling approximately 250 days of data collection from 2019 to 2021 — about a quarter of the total data expected from KATRIN,” said Dr. Kathrin Valerius, one of the two co-spokespersons of the KATRIN experiment and a physicist at the Karlsruhe Institute of Technology.
“With each campaign, we have gained new insights and further optimized the experimental conditions,” said Dr. Susanne Mertens, a physicist at the Max Planck Institute for Nuclear Physics and the Technical University Munich.
_____
Max Aker et al. (KATRIN Collaboration). 2025. Direct neutrino-mass measurement based on 259 days of KATRIN data. Science 388 (6743): 180-185; doi: 10.1126/science.adq9592
