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

Abstract

We present upper limits on the production of heavy neutral leptons (HNLs) decaying to $\hat{I}¼\ddot{I}€$ 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 $2.0\times 10^{20}$ protons on target from the Fermilab Booster Neutrino Beam, which produces mainly muon neutrinos with an average energy of $\approx 800$ 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 $|U_{\mu 4}{|}^2$ of the extended PMNS mixing matrix in the range $|U_{\mu 4}{|}^2<(6.6$-$0.9)\times 10^{-7}$ for Dirac HNLs and $|U_{\mu 4}{|}^2<(4.7$-$0.7)\times 10^{-7}$ for Majorana HNLs, assuming HNL masses between $260$ and $385$ MeV and $|U_{e4}{|}^2=|U_{\ddot{I}„4}{|}^2=0$.

11 pages, 9 figures. Final accepted version by Phys. Rev. D, minor textual changes