Search for heavy neutral leptons decaying into muon-pion pairs in the MicroBooNE detector

We present upper limits on the production of heavy neutral leptons (HNLs) decaying to μπ pairs using data collected with the MicroBooNE liquid-argon time projection chamber (TPC) operating at Fermilab. This search is the first of its kind performed in a liquid-argon TPC. We use data collected in 2017 and 2018 corresponding to an exposure of 2.0×10²⁰ protons on target from the Fermilab Booster Neutrino Beam, which produces mainly muon neutrinos with an average energy of ≈800 MeV. HNLs with higher mass are expected to have a longer time of flight to the liquid-argon TPC than Standard Model neutrinos. The data are therefore recorded with a dedicated trigger configured to detect HNL decays that occur after the neutrino spill reaches the detector. We set upper limits at the 90% confidence level on the element |U_μ4|² of the extended PMNS mixing matrix in the range |U_μ4|²<(6.6--0.9)×10^-7 for Dirac HNLs and |U_μ4|²<(4.7--0.7)×10^-7 for Majorana HNLs, assuming HNL masses between 260 and 385 MeV and |Uₑ₄|²=|U_τ4|²=0.