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 x 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 x 10(-8) for Majorana HNLs with a mass of mHNL = 246 MeV to |U-mu 4|(2) < 0.92 x 10(-8) for mHNL = 385 MeV, assuming |U-e4|(2) = |U-mu 4|(2) = 0 and HNL decays into mu(+/-)pi(-/+) 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. 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 x 10(-9) for mHPS = 212 GeV and.2 < 1.09 x 10(-9) for mHPS = 275 GeV. These are the first constraints on the scalar-Higgs mixing angle. from a dedicated experimental search in this mass range.

Automated summary

In this study, a search for long-lived Higgs portal scalars (HPS) and heavy neutral leptons (HNL) was conducted using data collected in the MicroBooNE liquid-argon time projection chamber. The results provide constraints on the mixing parameter and scalar-Higgs mixing angle, with improved sensitivity compared to previous studies.