KATRIN: A next generation tritium beta decay experiment with sub-eV sensitivity for the electron neutrino mass

TL;DR

The paper motivates measuring the absolute electron-neutrino mass and proposes KATRIN, a next-generation tritium beta-decay experiment employing a MAC-E-Filter with two independent tritium sources, a large main spectrometer, and MAC-E-TOF capabilities to reach sub-eV sensitivity. It details a comprehensive, technically demanding design including WGTS and QCTS sources, a 7 m-diameter spectrometer, advanced vacuum and HV systems, and a low-background detector, with careful control of systematics such as inelastic scattering. Sensitivity studies indicate a potential limit around 0.35 eV for the electron neutrino mass after about three years, enabling meaningful cosmological implications through the neutrino density parameter Ων and providing essential complements to oscillation and neutrinoless double beta decay experiments. If realized, KATRIN would constitute an important bridge between laboratory measurements and cosmology, refining our understanding of the absolute neutrino mass scale and informing theories beyond the Standard Model.

Abstract

With the compelling evidence for massive neutrinos from recent neutrino-oscillation experiments, one of the most fundamental tasks of particle physics over the next years will be the determination of the absolute mass scale of neutrinos. The absolute value of neutrino-masses will have crucial implications for cosmology, astrophysics and particle physics. We present the case for a next generation tritium beta decay experiment to perform a high precision direct measurement of the absolute mass of the electron neutrino with sub-eV sensitivity. We discuss the experimental requirements and technical challenges of the proposed Karlsruhe Tritium Neutrino experiment (KATRIN) and outline its physics potential.

Figures (18)

• Figure 2: The electron energy spectrum of tritium $\beta$ decay : (a) complete and (b) narrow region around endpoint $E_0$ . The $\beta$ spectrum is shown for neutrino masses of 0 and 1 eV.
• Figure 3: Bounds on the effective $\beta$ decay mass $m(\hbox{$\nu_{e}$})$ ($=m_{\beta}$ in ref. smirnov) as functions of the solar mixing angle $\theta_{\odot}$ (for 3$\nu$ mixing models with strong mass degeneracy). The horizontal lines correspond to the current and the proposed future tritium $\beta$ decay experiments (Mainz and KATRIN) as well as to structure formation. For results from $0\nu \beta \beta$ experiments, which strongly depend on $\theta_{\odot}$, the upper solid (dashed) line corresponds to the present bound $m_{ee}\leq 0.34$ eV and $|U_{e3}|^2=0$ ($|U_{e3}|^2$=0.05), the lower solid (dashed) line corresponds to an envisaged future limit of $m_{ee} \leq 0.05$ eV and $|U_{e3}|^2=0$ ($|U_{e3}|^2$=0.05). The vertical lines mark the current 90 % C.L. borders of the large mixing angle (LMA) solution region for solar $\nu$'s. (fig. taken from ref. smirnov)
• Figure 4: Results of tritium $\beta$ decay experiments on the observable $m_\nu^2$ over the last decade. The experiments at Los Alamos, Zürich, Tokyo, Beijing and Livermore LANLZuerichTokyoBejingLLNL used magnetic spectrometers, the experiments at Troitsk and Mainz Lobashev99Weinheimer99 are using electrostatic spectrometers of the MAC-E-Filter type (see text).
• Figure 5: Principle of the MAC-E-Filter. (a) Experimental setup, (b) Momentum transformation due to adiabatic invariance of magnetic orbit momentum $\mu$ in the inhomogeneous magnetic field.
• Figure 6: Schematic view of the Troitsk experimental setup Belesev95.