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

Blake, A. and Devitt, A. and Nowak, J. and Patel, N. and Thorpe, C. (2022) Search for long-lived heavy neutral leptons and Higgs portal scalars decaying in the MicroBooNE detector. Physical Review D, 106 (9): 092006. ISSN 1550-7998

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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 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 |Uμ4|2 ranging from |Uμ4|2<12.9×10-8 for Majorana HNLs with a mass of mHNL=246 MeV to |Uμ4|2<0.92×10-8 for mHNL=385 MeV, assuming |Ue4|2=|Uτ4|2=0 and HNL decays into μ±π pairs. These limits on |Uμ4|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.