KATRIN experiment sets most precise upper limit on neutrino mass: 0.45 eV

Researchers from the KATRIN (Karlsruhe Tritium Neutrino) 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. The findings tighten the constraints on one of the universe’s most elusive fundamental particles and push the boundaries of physics beyond the Standard Model. Neutrinos – electrically neutral elementary particles – 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. Here, Max Aker and 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. 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 Collaboration 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. “The neutrino mass measuring campaign of the KATRIN experiment will end in 2025 after reaching 1000 days of data acquisition,” writes Loredana Gastaldo in a related Perspective. “Analysis of the full data set gained from this grand project will allow for estimating the effective electron neutrino mass close to the projected value of 0.3 eV at 90% confidence level.”