The problem
When radioactive tritium decays, it emits an electron and an antineutrino, a particle that usually escapes without interacting. They share the released energy. How much mass do neutrinos actually have?
KATRIN measures the electrons. Near the highest possible electron energy, a neutrino’s mass changes the shape of the spectrum—the number of electrons counted at each energy. Oscillations, in which neutrinos change their interaction type as they travel, reveal differences between squared masses. Those differences do not fix the absolute masses, just as knowing a difference between two weights does not tell you either weight.[1] [2]
The electron energies emitted by tritium beta decay depend slightly on the mass of the escaping antineutrino. Can that spectrum determine the absolute mass scale, beyond the squared-mass differences known from neutrino oscillations?
KATRIN counts electrons that pass an adjustable energy filter near the decay endpoint. It fits the spectral shape while allowing for backgrounds, energy calibration, scattering in the source and the daughter molecule’s possible final states. A mass effect must survive these uncertainties.
The unresolved mass states contribute with their electron-flavour mixing weights. The measured squared effective mass is the weighted average of their squared masses; taking its square root gives the effective electron-neutrino mass commonly quoted here. It is different from the sum of masses probed by cosmological models. The cited five-campaign KATRIN analysis gives an upper bound of 0.45 eV/c² at 90% confidence, rather than a detection at that value.[2]
Scope
Determining the absolute neutrino mass scale, especially through laboratory beta-decay measurements, while distinguishing those measurements from cosmology-dependent constraints.
What remains open
Improved sensitivity must distinguish a mass-dependent spectral change from uncertainties. Cosmological constraints depend on modeling assumptions and should not be presented as equivalent to a direct laboratory detection.[2]
Why it matters
Neutrino masses influence the transition of relic neutrinos from fast-moving to slower matter and thereby the evolution of cosmic structure. Laboratory decay kinematics tests the scale without assuming a cosmological model. Comparing it with sky-based constraints therefore tests the assumptions behind those constraints as well as the unusually small masses that particle-mass theories must explain.[1] [2]
Definitions
3 terms
beta decay
A nuclear transformation that emits an electron and an antineutrino in the tritium example. Their energy distribution, including the daughter system’s recoil and final states, can constrain neutrino mass.[1] [2]
endpoint
The highest electron energy allowed by the decay kinematics. Electrons close to this limit carry the most useful information about the escaping neutrino’s mass.[2]
effective electron-neutrino mass
The square root of the average of the squared mass-state masses, weighted by their electron-flavour contributions. It is the unresolved beta-decay quantity, not an individual mass or the sum used in cosmology.[2]
Sources
- Astroparticle physics: Neutrinos weigh less than 0.45 electronvolts
Notes on Astroparticle physics: Neutrinos weigh less than 0.45 electronvolts
KIT explains tritium electron-energy spectroscopy, the cited 0.45 eV/c² effective-mass upper bound and its particle/cosmological relevance. This is an upper bound from specified analyzed data, not a neutrino mass measured at that value.
- Direct neutrino-mass measurement based on 259 days of KATRIN data
Notes on Direct neutrino-mass measurement based on 259 days of KATRIN data
The exact v1 primary KATRIN manuscript defines the effective squared mass, beta-decay endpoint measurement and systematics, and reports the 0.45 eV upper bound at 90% confidence for five campaigns. Its later 2025 Science publication is distinct from this 2024 manuscript locator.