Improved eV-scale sterile-neutrino constraints from the second KATRIN measurement campaign

We present the results of the light sterile neutrino search from the second Karlsruhe Tritium Neutrino (KATRIN) measurement campaign in 2019. Approaching nominal activity, $3.76\ifmmode\times\else\texttimes\fi{}{10}^{6}$ tritium $\ensuremath{\beta}$-electrons are analyzed in an energy window extending down to 40 eV below the tritium end point at ${E}_{0}=18.57\text{ }\text{ }\mathrm{keV}$. We consider the $3\ensuremath{\nu}+1$ framework with three active and one sterile neutrino flavors. The analysis is sensitive to a fourth mass eigenstate ${m}_{4}^{2}\ensuremath{\lesssim}1600\text{ }\text{ }{\mathrm{eV}}^{2}$ and active-to-sterile mixing $|{U}_{e4}{|}^{2}\ensuremath{\gtrsim}6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$. 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 $\mathrm{\ensuremath{\Delta}}{m}_{41}^{2}$ 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 $\mathrm{\ensuremath{\Delta}}{m}_{41}^{2}$ are mostly excluded by short-baseline reactor experiments, KATRIN is the only ongoing laboratory experiment to be sensitive to relevant solutions at large $\mathrm{\ensuremath{\Delta}}{m}_{41}^{2}$ through a robust spectral shape analysis.