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 NuMI neutrino beam from Fermilab’s Main Injector with a total exposure corresponding to $7.01\times {10}^{20}$ protons on target. We set upper limits at the $90\mathrm{\%}$ confidence level on the mixing parameter $\mid U_{\mu 4}{\mid }^2$ ranging from $\mid U_{\mu 4}{\mid }^2<12.9\times {10}^{-8}$ for Majorana HNLs with a mass of $m_{\mathrm{H}\mathrm{N}\mathrm{L}}=246$ MeV to $\mid U_{\mu 4}{\mid }^2<0.92\times {10}^{-8}$ for $m_{\mathrm{H}\mathrm{N}\mathrm{L}}=385$ MeV, assuming $\mid U_{e4}{\mid }^2=\mid U_{\tau 4}{\mid }^2=0$ and HNL decays into ${\mu }^{\pm }{\pi }^{\mp }$ pairs. These limits on $\mid U_{\mu 4}{\mid }^2$ represent an order of magnitude improvement in sensitivity compared to the previous MicroBooNE result. We also constrain the scalar-Higgs mixing angle $\theta$ by searching for HPS decays into ${\mu }^{+}{\mu }^{-}$ final states, excluding a contour in the parameter space with lower bounds of ${\theta }^2<31.3\times {10}^{-9}$ for $m_{\mathrm{H}\mathrm{P}\mathrm{S}}=212$ GeV and ${\theta }^2<1.09\times {10}^{-9}$ for $m_{\mathrm{H}\mathrm{P}\mathrm{S}}=275$ GeV. These are the first constraints on the scalar-Higgs mixing angle $\theta$ from a dedicated experimental search in this mass range.