Improved eV-scale Sterile-Neutrino Constraints from the Second KATRIN Measurement Campaign

TL;DR

The second KATRIN measurement campaign analyzed tritium beta decay data to search for sterile neutrinos, setting new exclusion limits on their mass and mixing parameters, and excluding large $ ext{ extDelta}m_{41}^2$ solutions related to reactor and gallium anomalies.

Contribution

This work provides the first constraints on sterile neutrinos from the second KATRIN campaign, surpassing previous laboratory limits and excluding key parameter regions related to anomalies.

Findings

No sterile neutrino signal was observed.

Improved exclusion contours on $m_4^2$ and $|U_{e4}|^2$ at 95% CL.

Excluded large $ ext{ extDelta}m_{41}^2$ solutions of reactor and gallium anomalies.

Abstract

We present the results of the light sterile neutrino search from the second 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 endpoint at $E_0 = 18.57\,$keV. We consider the $3\nu+1$ framework with three active and one sterile neutrino flavor. The analysis is sensitive to a fourth mass eigenstate $m_4^2\lesssim1600\,$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 $\Delta m_{41}^2$ 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 $\Delta m_{41}^2$ are mostly excluded by short-baseline reactor experiments, KATRIN is the only ongoing laboratory experiment to be sensitive to relevant solutions at large $\Delta m_{41}^2$ through a robust spectral shape analysis.