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

Abstract

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 $90\%$ confidence level on the mixing parameter $|U_{\hat{I}¼4}{|}^2$ ranging from $|U_{\hat{I}¼4}{|}^2<12.9\times 10^{-8}$ for Majorana HNLs with a mass of $m_{\mathrm{HNL}}=246$ MeV to $|U_{\hat{I}¼4}{|}^2<0.92\times 10^{-8}$ for $m_{\mathrm{HNL}}=385$ MeV, assuming $|U_{e4}{|}^2=|U_{\ddot{I}„4}{|}^2=0$ and HNL decays into $\hat{I}{¼}^{\pm }\ddot{I}{€}^{\mp }$ pairs. These limits on $|U_{\hat{I}¼4}{|}^2$ represent an order of magnitude improvement in sensitivity compared to the previous MicroBooNE result. We also constrain the scalar-Higgs mixing angle $\hat{I}¸$ by searching for HPS decays into $\hat{I}{¼}^{+}\hat{I}{¼}^{-}$ final states, excluding a contour in the parameter space with lower bounds of $\hat{I}{¸}^2<31.3\times 10^{-9}$ for $m_{\mathrm{HPS}}=212$ GeV and $\hat{I}{¸}^2<1.09\times 10^{-9}$ for $m_{\mathrm{HPS}}=275$ GeV. These are the first constraints on the scalar-Higgs mixing angle $\hat{I}¸$ from a dedicated experimental search in this mass range.

This paper is dedicated to the memory of Salvatore Davide Porzio. This is the journal accepted version with only minor presentational changes compared to preprint