Bound on 3+1 active-sterile neutrino mixing from the first four-week science run of KATRIN

Understand

We report on the light sterile neutrino search from the first four-week science run of the KATRIN experiment in~2019.

• Beta-decay electrons from a high-purity gaseous molecular tritium source are analyzed by a high-resolution MAC-E filter down to 40 eV below the endpoint at 18.57 keV.
• We consider the framework with three active neutrinos and one sterile neutrino of mass $m_{4}$.
• The analysis is sensitive to a fourth mass state $m^2_{4} \lesssim$ 1000 eV$^2$ and to active-to-sterile neutrino mixing down to $|U_{e4}|^2 \gtrsim 2\cdot10^{-2}$.