Precision measurement of the energy loss of e−-T2 scattering in the source of KATRIN

The Karlsruhe Tritium Neutrino experiment (KATRIN) is targeted
to measure the effective neutrino mass 𝑚(𝜈𝑒) with a sensitivity of
0.2 eV at 90 % CL. In order to determine the neutrino mass, an integrated
𝛽-decay spectrum close to the endpoint is measured and a fit to
the data comprising 𝑚(𝜈𝑒) as a free parameter is performed. A number
of systematic effects on the measured spectrum need to be taken into
account in the spectral analysis. One of these is the energy loss of the
𝛽-electrons from inelastic scatterings with the source gas. The probability
distribution of the energy losses of 18.6 keV electrons scattering
off molecular tritium can be determined in-situ with KATRIN by using
a pulsed monoenergetic source of photoelectrons, allowing for time-offlight
spectroscopy. The measurement is analysed by fitting a newly
developed, semi-empirical energy-loss model to a combined data set of
integral and differential spectra. With this measurement and analysis
technique, an analytical description of the energy loss function with
unprecedented precision is obtained. In this contribution, the latest
analysis results will be presented and discussed in the context of the
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neutrino-mass systematics budget of KATRIN.
This work is supported by the Helmholtz Association (HGF), the
Ministry for Education and Research BMBF (05A17PM3, 05A17PX3,
05A17VK2, and 05A17WO3), and the Helmholtz Alliance for Astroparticle
Physics (HAP).