Bound on 3 +1 Active-Sterile Neutrino Mixing from the First Four-Week Science Run of KATRIN

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. The analysis is sensitive to the mass, m₄, of the fourth mass state for m₄²≲1000 eV² and to active-to-sterile neutrino mixing down to |U_{e 4}|²≳2 ×10^-2. No significant spectral distortion is observed and exclusion bounds on the sterile mass and mixing are reported. These new limits supersede the Mainz results for m₄²≲1000 eV² and improve the Troitsk bound for m₄²<30 eV². The reactor and gallium anomalies are constrained for 100 <Δ m₄₁²<1000 eV².