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Glossary · K

Kinematic neutrino mass

The model-independent effective mass measured by tritium β-decay endpoint experiments such as KATRIN.

Also: m_β · effective electron-neutrino mass · tritium endpoint mass

Definition

mβ2 = Σ |Uei|² mi² is the incoherent weighted sum of the squared mass eigenstates that appears in the β-decay phase-space factor near the endpoint. It is fundamentally distinct from the cosmological Σmν (which is the simple sum) and from the Majorana ⟨mββ⟩ (which depends on Majorana phases).

The kinematic limit is model-independent — no Majorana / Dirac assumption, no cosmological-model assumption — which makes it the most defensible bound on the absolute neutrino-mass scale. Current value: mβ < 0.45 eV from KATRIN combined KNM1–5 (2024); design goal < 0.2 eV expected by KNM1–6.

See also

  • Effective Majorana mass The coherent sum of neutrino mass eigenstates weighted by PMNS factors that controls the rate of 0νββ decay.
  • Mass eigenstate A neutrino state with a definite mass m_i; the propagation basis (i = 1, 2, 3).
  • Cosmic neutrino background A predicted relic background of low-energy neutrinos that decoupled from the primordial plasma about one second after the Big Bang.

References

  1. KATRIN Collab., Nature Phys. 18, 160 (2022)

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Formula

Beta-decay endpoint

How KATRIN reads the absolute neutrino mass off the spectrum.

Glossary

Beta decay

A nuclear weak-interaction process that converts a neutron to a proton (β⁻) or vice versa (β⁺), emitting an electron/positron and a (anti)neutrino.

Experiment

KATRIN

World's tightest direct neutrino-mass limit: m_β < 0.45 eV.