We present the results of the light sterile neutrino search from the second Karlsruhe Tritium Neutrino (KATRIN) measurement campaign in 2019. Approaching nominal activity, $3.76\times {10}^6$ tritium $\beta$-electrons are analyzed in an energy window extending down to 40 eV below the tritium end point at $E_0=18.57\mathrm{keV}$. We consider the $3\nu +1$ framework with three active and one sterile neutrino flavors. The analysis is sensitive to a fourth mass eigenstate $m_4^2\lesssim 1600{\mathrm{eV}}^2$ and active-to-sterile mixing $|U_{e4}{|}^2\gtrsim 6\times {10}^{-3}$. As no sterile-neutrino signal was observed, we provide improved exclusion contours on $m_4^2$ and $|U_{e4}{|}^2$ at 95% C.L. Our results supersede the limits from the Mainz and Troitsk experiments. Furthermore, we are able to exclude the large $\mathrm{\Delta }{m}_{41}^2$ solutions of the reactor antineutrino and gallium anomalies to a great extent. The latter has recently been reaffirmed by the BEST Collaboration and could be explained by a sterile neutrino with large mixing. While the remaining solutions at small $\mathrm{\Delta }{m}_{41}^2$ are mostly excluded by short-baseline reactor experiments, KATRIN is the only ongoing laboratory experiment to be sensitive to relevant solutions at large $\mathrm{\Delta }{m}_{41}^2$ through a robust spectral shape analysis.

• Received 2 February 2022
• Accepted 24 March 2022

DOI:https://doi.org/10.1103/PhysRevD.105.072004

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Published by the American Physical Society