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

TL;DR

This paper reports a search for long-lived heavy neutral leptons and Higgs portal scalars decaying in the MicroBooNE detector, setting new limits on their properties and improving sensitivity over previous results.

Contribution

First experimental constraints on scalar-Higgs mixing angle and heavy neutral leptons in the specified mass ranges using MicroBooNE data.

Findings

Set upper limits on heavy neutral lepton mixing parameter $ vert U_{\mu 4} vert^2$.

Excluded a parameter space region for scalar-Higgs mixing angle $ heta$.

Improved sensitivity compared to previous MicroBooNE results.

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 $\lvert U_{\mu 4}\rvert^2$ ranging from $\lvert U_{\mu 4}\rvert^2<12.9\times 10^{-8}$ for Majorana HNLs with a mass of $m_{\rm HNL}=246$ MeV to $\lvert U_{\mu 4}\rvert^2<0.92 \times 10^{-8}$ for $m_{\rm HNL}=385$ MeV, assuming $\lvert U_{e 4}\rvert^2 = \lvert U_{\tau 4}\rvert^2 = 0$ and HNL decays into $\mu^\pm\pi^\mp$ pairs. These limits on $\lvert U_{\mu 4}\rvert^2$ 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 $m_{\rm HPS}=212$ GeV and $\theta^2<1.09 \times 10^{-9}$ for $m_{\rm HPS}=275$ GeV. These are the first constraints on the scalar-Higgs mixing angle $\theta$ from a dedicated experimental search in this mass range.