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 $\ensuremath{\mu}\ensuremath{\pi}$ 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 $2.0\ifmmode\times\else\texttimes\fi{}{10}^{20}$ protons on target from the Fermilab Booster Neutrino Beam, which produces mainly muon neutrinos with an average energy of $\ensuremath{\approx}800\text{ }\text{ }\mathrm{MeV}$. 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 $|{U}_{\ensuremath{\mu}4}{|}^{2}$ of the extended PMNS mixing matrix in the range $|{U}_{\ensuremath{\mu}4}{|}^{2}<(6.6--0.9)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$ for Dirac HNLs and $|{U}_{\ensuremath{\mu}4}{|}^{2}<(4.7--0.7)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$ for Majorana HNLs, assuming HNL masses between 260 and 385 MeV and $|{U}_{e4}{|}^{2}=|{U}_{\ensuremath{\tau}4}{|}^{2}=0$.