KATRIN experiment

TL;DR

The paper surveys the KATRIN experiment, a direct and model-independent probe of the electron neutrino mass via the endpoint region of tritium beta decay. It details the instrument design—windowless gaseous tritium source (WGTS), MAC-E filter spectrometer, and a segmented focal-plane detector—and the data-analysis framework that convolves the theoretical beta spectrum with a measured detector response to extract $m_ u^2$. It reports that KNM1–KNM5 yield $m^2_ u = -0.14^{+0.13}_{-0.15}$ eV$^2$ and $m_ u < 0.45$ eV (90% C.L.), with final sensitivity targeted below 0.3 eV, and discusses complementary searches for sterile neutrinos and other BSM phenomena. Looking ahead, phase 2 (TRISTAN) will probe keV-scale sterile neutrinos and KATRIN++ aims to reach 50 meV (and possibly 9 meV) sensitivity using differential measurements and atomic-tritium sources, potentially resolving the neutrino-mass ordering found in oscillation experiments and informing cosmological analyses.

Abstract

Since the discovery of neutrino oscillations, it is known that neutrinos have small but non-zero masses. The neutrino mass scale, which is of fundamental importance for cosmology, astrophysics and particle physics, can be measured directly from the kinematics of weak decays. The Karlsruhe tritium neutrino experiment KATRIN measures the end point region of the tritium \b{eta}-spectrum with unrivalled t statistics and an unprecedented precision. This world-leading direct neutrino mass search experiment is characterised by a windowless, gaseous molecular tritium source and a giant MAC- E filter-type spectrometer. The precision measurement of the tritium \b{eta}-spectrum also allows the search for many other phenomena beyond the Standard Model of particle physics. The KATRIN experiment is about to reach its target sensitivity of the neutrino mass of less than 300 meV and will then turn its attention to the search for sterile keV neutrinos before the neutrino mass sensitivity is to be significantly increased once again by applying quantum read-out technology combined with an atomic tritium source with KATRIN++.

Figures (17)

• Figure 1: A view inside the huge KATRIN spectrometer during installing the wire electrode system (photo and copyright: Michael Zacher).
• Figure 2: Beta spectrum of tritium (left) and its endpoint region for two assumed neutrino masses of $m_\nu=0$ and $m_\nu=1$ eV (right), copyright Christian Weinheimer.
• Figure 3: Neutrino mass limits from direct neutrino mass experiments over the last 80 years quoting the different technologies and isotopes used. The references on neutrino mass limits from tritium are Formaggio:2021nfzProject8:2022hunPhysRevD.110.030001KATRIN:2024cdt, the limits from $^{187}$Re are from references PhysRevD.110.030001Camilleri:2008zzGatti:2001ty, and those from $^{163}$Ho are from references Camilleri:2008zzVelte:2019jvxAlpert:2025tqqECHo:2025ook (courtesy and copyright: Magnus Schlösser).
• Figure 4: The KATRIN experiment with its major components from left to right: the calibration and monitoring system with its rear wall end electron gun, the windowless gaseous molecular tritium source WGTS, the electron transport section composed of a differential and a cryogenic pumping system, the small pre- and the large main spectrometer in the so-called SAP configuration, and the segmented detector KATRIN:2024cdt_arXiv (license https://creativecommons.org/licenses/by/4.0/) , see also KATRIN:2024cdt.
• Figure 5: MAC-E filter at the example of the KATRIN main spectrometer in the normal, symmetric analysis plane configuration. A superconducting solenoid at the entrance and a superconducting solenoid at the exit together with the aircoil system form the magnetic field $B$ (field lines in grey and blue), whereas the electric potential at the spectrometer vessel (black) and the inner electrode system (red) form the retarding potential $U_\mathrm{max}$ (field lines in pink). The analysis plane is illustrated in the middle with the conditions $B=B_\mathrm{min}$ and $-|U| = -|U_\mathrm{max}|$. Tracks of a transmitted, a reflected and a stored electron are illustrated in green. The third superconducting magnet housing the detector is exhibited as well. The diagram below illustrates the transmation of the momentum vector of the electrons according Eq. \ref{['eq:adiabatic_invariant']} neglecting the electric retarding potential KATRIN:2021dfa (license https://creativecommons.org/licenses/by/4.0/) .