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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 × 10 20 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 jU μ4 j 2 of the extended PMNS mixing matrix in the range jU μ4 j 2 < ð6.6-0.9Þ × 10 −7 for Dirac HNLs and jU μ4 j 2 < ð4.7-0.7Þ × 10 −7 for Majorana HNLs, assuming HNL masses between 260 and 385 MeV and jU e4 j 2 ¼ jU τ4 j 2 ¼ 0.

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