Probing the Higgs Portal at the Fermilab Short-Baseline Neutrino Experiments

TL;DR

This work evaluates the sensitivity of Fermilab's Short-Baseline Neutrino (SBN) experiments to light Higgs-portal scalars with masses in the MeV–GeV range, produced mainly via kaon decays in the Booster and NuMI beams. It employs Geant4-based beam simulations (g4bnb/g4numi) and GENIE v3 for neutrino backgrounds to estimate signals S→e^+e^- and S→μ^+μ^-/π^+π^- in the SBND, MicroBooNE, and ICARUS detectors, including detector effects and timing considerations. The study finds that SBND offers the best Booster-based sensitivity while ICARUS dominates off-axis NuMI production, collectively enabling leading searches for sub-GeV scalars with mixing angles θ down to a few ×10^-5 in the 50–350 MeV mass window. These results motivate dedicated experimental searches for dark scalars and highlight timing and KDAR-based strategies as promising tools within the SBN program.

Abstract

The Fermilab Short-Baseline Neutrino (SBN) experiments, MicroBooNE, ICARUS, and SBND, are expected to have significant sensitivity to light weakly coupled hidden sector particles. Here we study the capability of the SBN experiments to probe dark scalars interacting through the Higgs portal. We investigate production of dark scalars using both the Fermilab Booster 8 GeV and NuMI 120 GeV proton beams, simulating kaons decaying to dark scalars and taking into account the beamline geometry. We also investigate strategies to mitigate backgrounds from beam-related neutrino scattering events. We find that SBND, with its comparatively short ${\cal O}(100\ {\rm m})$ baseline, will have the best sensitivity to scalars produced with Booster, while ICARUS, with its large detector volume, will provide the best limits on off-axis dark scalar production from NuMI. The SBN experiments can provide leading tests of dark scalars with masses in the 50 - 350 MeV range in the near term. Our results motivate dedicated experimental searches for dark scalars and other long-lived hidden sector states at these experiments.

Figures (16)

• Figure 1: Left: Scalar branching ratios. Right: Isocontours of scalar decay length in m in the $m_S - \theta$ plane.
• Figure 2: Dominant signal production process via kaon decays.
• Figure 3: Coordinates in the "BNB frame" defined by a left-handed coordinate system with the origin at the BNB target, the $z$-axis pointing to center of the SBN detectors, and the $y$-axis pointing vertically upwards. We show the coordinates of the BNB and NuMI targets, NuMI absorber, and the SBND, MicroBooNE, and ICARUS detectors in both aerial (left) and elevation (right) views.
• Figure 4: Kinematics of kaons produced per POT by g4bnb (left) and g4numi (right) simulations, at the time of their decays to neutrinos. The angle is taken with respect to the beamline. Kaons that stop before decaying are not shown above; see the text for details.
• Figure 5: Energies of scalars produced by g4bnb that decay in SBND (left) and by g4numi that decay in ICARUS (right). There are peaks at low $p$ in the $K^+$ distributions from KDAR.