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 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[subscript µ4]|² of the extended PMNS mixing matrix in the range |U[subscript µ4]|²<(6.6–0.9)×10-⁷ for Dirac HNLs and |U[subscript µ4]|²<(4.7–0.7)×10-⁷ for Majorana HNLs, assuming HNL masses between 260 and 385 MeV and |U[subscript e4]|²=|U[subscript t4]|²=0. ©2020