Speaker
Description
Heavy neutral leptons (HNLs) are hypothetical particles that could help explain the origin of neutrino masses. Their interactions with Standard Model particles are suppressed by small active–sterile mixing, but a scalar mediator can open additional production channels. In this work, we study HNL production through scalar production in meson decays and proton bremsstrahlung, followed by scalar decays to HNLs and their subsequent visible decays. We calculate the resulting signal yields at the DUNE, T2K, and Hyper-K near detectors, comparing the contributions of these production mechanisms with conventional mixing-induced HNL production. We examine how these additional production mechanisms change the inferred experimental reach relative to mixing-induced HNL production alone. We also recast existing fixed-target HNL limits under the scalar-portal model, accounting for the added scalar-mediated production. We map the projected sensitivities and revised constraints across the HNL and scalar masses, active–sterile mixing, and portal couplings, and identify where scalar-mediated production substantially changes the HNL parameter space that can be probed experimentally. The goal is to explore how non-minimal HNL models with scalar mediators could open up new possibilities and parameter spaces in combination with minimal model limit predictions.