KATRIN

Objective

Model-independent direct measurement of the electron-antineutrino effective mass from the shape of the tritium beta spectrum near its 18.6 keV endpoint.

Method

A windowless gaseous tritium source at 10¹¹ decays per second feeds electrons into a differential pumping and cryogenic trapping section, then into a 10 m × 23 m main spectrometer operating as a MAC-E filter. The retarding potential is scanned in small steps near the endpoint; transmitted electrons are counted by a segmented silicon detector. The integral-spectrum shape is fit for m(νe)².

Key results

• 2019: first commissioning results with preliminary upper limit on neutrino mass.
• 2022: m(νe) < 0.8 eV (90% C.L.), world-leading direct upper limit.
• 2024: m(νe) < 0.45 eV (90% C.L.) with three campaigns of data — world-leading.
• Design sensitivity 0.2 eV over 5 years of data.

Significance

KATRIN provides the most sensitive model-independent limit on the absolute neutrino mass scale. The measurement is complementary to cosmological bounds (which depend on ΛCDM assumptions) and to neutrinoless double beta decay limits (which assume Majorana nature). A KATRIN signal — or continued non-observation — anchors one corner of the multi-dimensional constraint on absolute mass, ordering, and Majorana phase.

The endpoint technique

The maximum kinetic energy an electron can carry in tritium beta decay is set by the Q-value of the transition minus the neutrino rest mass energy. A finite neutrino mass displaces the endpoint downward by $m_{{\nu }_e}{c}^2$ and distorts the spectrum shape in a narrow window just below. The fraction of decays within the last few eV of the endpoint scales as $(m_{\nu }/Q{)}^3$, heavily favoring low-$Q$ isotopes. Tritium, at $Q=18.574$ keV, is the most practical choice.

The apparatus

KATRIN comprises five sequential sections spanning 70 m of beamline:

1. Windowless Gaseous Tritium Source (WGTS) — a 10-meter-long beam pipe at 30 K with continuous tritium gas injection, producing ~$10^{11}$ β-decays per second with precisely controlled column density
2. Differential pumping section — suppresses tritium flow by a factor of $10^{14}$ while preserving electron transmission through axial magnetic fields
3. Cryogenic pumping section — traps residual tritium on a cold argon frost
4. Pre-spectrometer — a smaller MAC-E filter rejecting electrons below ~18 keV
5. Main spectrometer — the 10 m × 23 m, 150 kV retarding potential filter, under ultrahigh vacuum ($<10^{-11}$ mbar), producing an energy resolution of 1 eV at the endpoint
6. Focal-plane detector — a 148-pixel silicon detector counting the transmitted electrons

The MAC-E (Magnetic Adiabatic Collimation with Electrostatic filtering) principle uses a gradient of magnetic field strength to align electron momenta to the axis, so the electrostatic filter measures total kinetic energy rather than just the axial component.

Key systematic challenges

KATRIN’s statistical goal of 0.2 eV sensitivity demands control of many systematic effects:

• High-voltage stability to 3 parts in $10^6$ on the retarding potential
• Magnetic-field uniformity and alignment
• Electronic and rotational-vibrational final states of the ${}^3$HeT${}^{+}$ daughter ion
• Tritium column-density stability and isotopologue composition
• Source-plasma effects on the electron energy

An extensive calibration program, including a dedicated gaseous ${}^{83m}$Kr source with a sharp monoenergetic conversion line at 17.8 keV, measures many of these in situ.

Results

KATRIN’s published results have progressively tightened the neutrino mass limit:

• KNM1 (first science run, 2019): $m_{{\nu }_e}<1.1$ eV
• KNM2 (2020): $m_{{\nu }_e}<0.9$ eV
• KNM1+2 combined (2022): $m_{{\nu }_e}<0.8$ eV
• KNM1+2+3+4+5 combined (2024): $m_{{\nu }_e}<0.45$ eV at 90% C.L.

All limits are world-leading model-independent bounds. The effective mass measured is $m_{{\nu }_e}^{\text{eff}}\equiv \sqrt{{\sum }_i|U_{ei}{|}^2{m}_i^2}$ an incoherent sum weighted by the PMNS first row.

Future

KATRIN is designed to continue through 2025 with its full dataset targeting 0.2 eV sensitivity. Beyond KATRIN, Project 8 uses cyclotron radiation emission spectroscopy to measure single tritium decays in a magnetic trap, with the goal of sub-40-meV sensitivity — below the minimum allowed by inverted ordering. Atomic tritium sources would remove the molecular final-state systematics that currently limit KATRIN.