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

Abstract

We present the first search for heavy neutral leptons (HNL) decaying into $\hat{I}½e^{+}{e}^{-}$ or $\hat{I}½\ddot{I}{€}^0$ 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 $|U_{\hat{I}¼4}{|}^2$ in the mass ranges $10\le m_{\mathrm{HNL}}\le 150$ MeV for the $\hat{I}½e^{+}{e}^{-}$ channel and $150\le m_{\mathrm{HNL}}\le 245$ MeV for the $\hat{I}½\ddot{I}{€}^0$ channel, assuming $|U_{e4}{|}^2=|U_{\ddot{I}„4}{|}^2=0$. These limits represent the most stringent constraints in the mass range $35<m_{\mathrm{HNL}}<175$ MeV and the first constraints from a direct search for $\hat{I}½\ddot{I}{€}^0$ decays.

Version as accepted by Physical Review Letters, some presentational changes and updated references, no changes to results