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 (HNLs) decaying into νe⁺e⁻ or νπ⁰ 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 7.01×10²⁰ protons on target. We set upper limits at the 90% confidence level on the mixing parameter |Uμ4|² in the mass ranges 10≤mHNL≤150 MeV for the νe⁺e⁻ channel and 150≤mHNL≤245 MeV for the νπ⁰ channel, assuming |Uₑ₄|²=|Uτ4|²=0. These limits represent the most stringent constraints in the mass range 35<mHNL<175 MeV and the first constraints from a direct search for νπ⁰ decays.