MicroBooNE experiment, NuMI absorber, and heavy neutral leptons

Motivated by the recent search for a Higgs portal scalar decaying inside the MicroBooNE detector, we demonstrate that the same search can be used to constrain heavy neutral leptons. These are gauge-singlet fermions that interact with the Standard Model by mixing with neutrinos only and could be related to the origin of neutrino masses. By recasting the results of the MicroBooNE Collaboration’s analysis, we show that, for a heavy neutral lepton that mixes predominantly with muon-flavored neutrinos, previously unexplored parameter space can be excluded for masses between 30 and 150 MeV. Additionally, we make our Monte Carlo tools publicly available.

Figure 1

MicroBooNE constraint on heavy neutral lepton parameter space as a function of its mass and mixing with muon neutrinos. Two assumptions regarding signal efficiency are shown in black (see text for detail) as well as comparisons against existing constraints [14, 20, 24, 47, 48, 58] (colored regions as labeled).

Figure 2

Relative flux (blue) and decay probabilities (red) between the heavy neutral lepton $N$ and the Higgs portal scalar $S$ coming from the NuMI absorber via two-body kaon decay-at-rest and decaying within MicroBooNE. The product of these two, in addition to relative signal efficiencies, provides the relative signal event rate in MicroBooNE.

Figure 3

Distributions of $e^{+}{e}^{-}$ pairs as a function of the cosine of their opening angle vs. the angle of the higher-energy particle relative to the NuMI absorber direction. Top: truth-level information, Bottom: reconstructed information after applying a $3°$ angular uncertainty on electron tracks. Blue/green points come from an HNL with $m_N=100\mathrm{MeV}$ where the two purple/orange ones come from HPS with $m_S=100\mathrm{MeV}$ (top) and $m_S=80\mathrm{MeV}$ (bottom). In all three, brighter/darker colors correspond to more signal events.

Figure 4

Our two assumed signal efficiencies for HNL reconstruction: that the HNL efficiency matches the HPS efficiency given in Ref. [12] (solid) and that the dependence of the efficiency is based on the average opening angle of the electron/positron pair (dashed).