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 $7.01\times {10}^{20}$ protons on target. We set upper limits at the 90% confidence level on the mixing parameter $|U_{\mu 4}{|}^2$ ranging from $|U_{\mu 4}{|}^2<12.9\times {10}^{-8}$ for Majorana HNLs with a mass of $m_{\mathrm{HNL}}=246\mathrm{MeV}$ to $|U_{\mu 4}{|}^2<0.92\times {10}^{-8}$ for $m_{\mathrm{HNL}}=385\mathrm{MeV}$, assuming $|U_{e4}{|}^2=|U_{\tau 4}{|}^2=0$ and HNL decays into ${\mu }^{\pm }{\pi }^{\mp }$ pairs. These limits on $|U_{\mu 4}{|}^2$ 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 $m_{\mathrm{HPS}}=212\mathrm{GeV}$ and ${\theta }^2<1.09\times {10}^{-9}$ for $m_{\mathrm{HPS}}=275\mathrm{GeV}$. These are the first constraints on the scalar-Higgs mixing angle $\theta$ from a dedicated experimental search in this mass range.

• Received 11 July 2022
• Accepted 31 October 2022

DOI:https://doi.org/10.1103/PhysRevD.106.092006

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Published by the American Physical Society