Origin of the discrepancy between integral and time-of-flight energy-loss measurements

Identify the origin of the discrepancy between the integral and time-of-flight calibration modes used to measure the electron energy-loss function in the KATRIN windowless gaseous tritium source and determine how this discrepancy affects the neutrino-mass inference.

Background

KATRIN relies on an accurate electron energy-loss function to model inelastic scattering in the tritium source. This function has been measured using two calibration modes (integral and time-of-flight) with a monoenergetic photoelectron source.

Recent calibrations revealed a discrepancy between these two modes that could bias the squared neutrino mass by up to 0.035 eV^2. As an interim measure, the analysis inflates the covariance of the energy-loss parameters, but the physical cause of the disagreement is not yet resolved.