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Short answer
KATRIN (the Karlsruhe Tritium Neutrino experiment) is a large international experiment at the Karlsruhe Institute of Technology (KIT) that measures the mass of the neutrino directly, from the beta decay of tritium. Its 2025 result is an upper limit: the neutrino mass is below 0.45 eV/c² at 90% confidence level.
Definition and scope
Neutrinos are among the lightest known particles, and KIT describes them as at least a million times lighter than electrons. KATRIN does not look for a neutrino itself. It uses the beta decay of tritium, an unstable hydrogen isotope, and records the energy distribution of the emitted electrons. Because that distribution depends on the neutrino mass through energy and momentum conservation alone, KIT calls the method a direct, model-independent kinematic determination (KIT press release 029/2025, read 2026-11-08). Results from cosmological observations, by contrast, depend on a cosmological model.
How it works
According to the experiment’s overview in the Encyclopedia of Particle Physics (Drexlin and Weinheimer, arXiv:2601.00248), KATRIN combines a windowless gaseous molecular tritium source with a very large MAC-E filter spectrometer, and the measurement is made in the region near the end of the electron spectrum, where a nonzero neutrino mass changes the spectral shape most. KIT reports a 70 m beamline with an intense tritium source and a high-resolution spectrometer 10 m in diameter. More than 20 institutions in 7 countries take part.
Results so far
- 2022 result: an earlier KATRIN limit, which the 2025 analysis improves by almost a factor of two (KIT).
- 2025 result: mν < 0.45 eV at 90% CL, from 259 measurement days covering the first five measurement campaigns (2019 to 2021). The data are reported in M. Aker et al. (KATRIN Collaboration), Science 388, 180-185 (2025), DOI 10.1126/science.adq9592; the arXiv version is 2406.13516. KIT states this is about a quarter of the data KATRIN expects to take in total.
- Target: the experiment overview states that KATRIN is about to reach its target sensitivity of a neutrino mass below 300 meV (0.3 eV).
Types or related concepts
Two approaches to neutrino mass are worth distinguishing, both as described by KIT. A direct, kinematic measurement such as KATRIN’s relies only on energy and momentum conservation in beta decay and is model-independent. Cosmological observations also constrain neutrino mass, but those results depend on a cosmological model. Separately, KATRIN’s planned TRISTAN detector targets a different question: sterile neutrinos in the keV range, hypothetical particles that are a dark-matter candidate (see What comes next).
Where it matters
KATRIN is a typical big-science collaboration: results come from a named collaboration with co-spokespersons and analysis coordinators rather than a small author list, which is why authorship and credit conventions for such groups matter. See CASRAI’s entries on group authorship, corporate authorship and co-authorship. For the surrounding field, see what is particle physics and what is physics.
Common misunderstandings
This is the most common misreading. A limit of 0.45 eV does not mean the neutrino weighs 0.45 eV. It means that, at 90% confidence, the effective mass probed by the spectrum is not larger than that. The true value may be far smaller, and a better experiment lowers the limit rather than confirming a number. The unit also trips readers up: eV/c² is a mass unit, and KIT gives the equivalent of 0.45 eV/c² as about 8 × 10-37 kg.
What comes next
KIT says neutrino-mass data taking continued to the end of 2025. From 2026 the TRISTAN detector system is to be installed to search for sterile neutrinos in the keV range, a hypothetical, even more weakly interacting particle that is a dark-matter candidate. KATRIN++ is the name for research and development toward a next-generation direct mass experiment; the overview paper mentions quantum read-out and an atomic tritium source as concepts.
FAQ
What does KATRIN stand for?
Karlsruhe Tritium Neutrino experiment. The name is also a given name, which is why searches for the bare word return unrelated results.
Where is KATRIN?
At KIT in Karlsruhe, Germany, with partner institutions in 7 countries.
What is the current KATRIN limit?
mν < 0.45 eV at 90% CL, published in Science in 2025 from 259 days of data.
Is KATRIN finished?
Neutrino-mass data taking was planned to continue to the end of 2025, with TRISTAN planned from 2026 (KIT).
Sources
- KIT Press Release 029/2025, Neutrinos weigh less than 0.45 electronvolts, read 2026-11-08.
- M. Aker et al. (KATRIN Collaboration), Direct neutrino-mass measurement based on 259 days of KATRIN data, arXiv:2406.13516 (abstract read 2026-11-08); Science 388, 180-185 (2025).
- G. Drexlin and C. Weinheimer, KATRIN experiment, arXiv:2601.00248, Encyclopedia of Particle Physics.
