We present the first search for heavy neutral leptons (HNLs) decaying into $\nu e^{+}{e}^{-}$ or $\nu {\pi }^0$ final states in a liquid-argon time projection chamber using data collected with the MicroBooNE detector. The data were recorded synchronously with the NuMI neutrino beam from Fermilab’s main injector corresponding to a total exposure of $7.01\times {10}^{20}$ protons on target. We set upper limits at the 90% confidence level on the mixing parameter $|U_{\mu 4}{|}^2$ in the mass ranges $10\le m_{\mathrm{HNL}}\le 150\mathrm{MeV}$ for the $\nu e^{+}{e}^{-}$ channel and $150\le m_{\mathrm{HNL}}\le 245\mathrm{MeV}$ for the $\nu {\pi }^0$ channel, assuming $|U_{e4}{|}^2=|U_{\tau 4}{|}^2=0$. These limits represent the most stringent constraints in the mass range $35<m_{\mathrm{HNL}}<175\mathrm{MeV}$ and the first constraints from a direct search for $\nu {\pi }^0$ decays.

• Received 12 October 2023
• Accepted 30 November 2023

DOI:https://doi.org/10.1103/PhysRevLett.132.041801

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Published by the American Physical Society