Search for long-lived heavy neutral leptons and Higgs portal scalars decaying in the MicroBooNE detector

We present a search for long-lived Higgs portal scalars (HPS) and heavy neutral leptons (HNL) decaying in the MicroBooNE liquid-argon time projection chamber. The measurement is performed using data collected synchronously with the neutrino beam from Fermilab's Main Injector with a total exposure corresponding to 7.01×10²⁰ protons on target. We set upper limits at the 90% confidence level on the mixing parameter |U_μ4|² ranging from |U_μ4|²<12.9×10^-8 for Majorana HNLs with a mass of mHNL=246 MeV to |U_μ4|²<0.92×10^-8 for mHNL=385 MeV, assuming |Uₑ₄|²=|U_τ4|²=0 and HNL decays into μ^±π^∓ pairs. These limits on |U_μ4|² represent an order of magnitude improvement in sensitivity compared to the previous MicroBooNE result. We also constrain the scalar-Higgs mixing angle θ by searching for HPS decays into μ⁺μ^- final states, excluding a contour in the parameter space with lower bounds of θ²<31.3×10^-9 for mHPS=212 GeV and θ²<1.09×10^-9 for mHPS=275 GeV. These are the first constraints on the scalar-Higgs mixing angle θ from a dedicated experimental search in this mass range.