Search for heavy neutral leptons in electron-positron and neutral-pion final states with the MicroBooNE detector

We present the first search for heavy neutral leptons (HNL) 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 $90\mathrm{\%}$ confidence level on the mixing parameter $\mid U_{\mu 4}{\mid }^2$ in the mass ranges $10\le m_{\mathrm{H}\mathrm{N}\mathrm{L}}\le 150$ MeV for the $\nu e^{+}{e}^{-}$ channel and $150\le m_{\mathrm{H}\mathrm{N}\mathrm{L}}\le 245$ MeV for the $\nu {\pi }^0$ channel, assuming $\mid U_{e4}{\mid }^2=\mid U_{\tau 4}{\mid }^2=0$. These limits represent the most stringent constraints in the mass range $35<m_{\mathrm{H}\mathrm{N}\mathrm{L}}<175$ MeV and the first constraints from a direct search for $\nu {\pi }^0$ decays.