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This article presents a measurement of νe interactions without pions in the final state using the MicroBooNE experiment and an investigation into the excess of low-energy electromagnetic events observed by the MiniBooNE collaboration. The measurement is performed in exclusive channels with (1eNp0π) and without (1e0p0π) visible final-state protons using 6.86×10 20 protons on target of data collected from the Booster Neutrino Beam at Fermilab. Events are reconstructed with the Pandora pattern recognition toolkit and selected using additional topological information from the MicroBooNE liquid argon time projection chamber. Using a goodness-of-fit test the data are found to be consistent with the predicted number of events with nominal flux and interaction models with a p-value of 0.098 in the two channels combined. A model based on the low-energy excess observed in MiniBooNE is introduced to quantify the strength of a possible νe excess. The analysis suggests that if an excess is present, it is not consistent with a simple scaling of the νe contribution to the flux. Combined, the 1eNp0π and 1e0p0π channels do not give a conclusive indication about the tested model, but separately they both disfavor the low-energy excess model at >90% CL. The observation in the most sensitive 1eNp0π channel is below the prediction and consistent with no excess. In the less sensitive 1e0p0π channel the observation at low energy is above the prediction, while overall there is agreement over the full energy spectrum.

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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 × 10 20 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 θ 2 < 31.3 × 10 −9 for mHPS = 212 GeV and θ 2 < 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.

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