Muon Bremsstrahlung as a New Probe of Dark Sector at Neutrino Experiments

Abstract

We show that muon bremsstrahlung provides a new production mechanism for light new physics at accelerator neutrino experiments beyond the meson kinematic limit. The intense, highly collimated muon beam produced alongside the neutrino beam in meson decays provides a powerful source for the bremsstrahlung production of new physics when it impinges on the beam dump at these facilities. As a benchmark scenario, we consider a muonphilic scalar coupled to Heavy Neutral Leptons (HNLs) and show that muon bremsstrahlung enables the production of HNLs with masses beyond the kinematic reach of meson decays. Using this new production mechanism and focusing on HNL mixing with tau neutrinos, we find that the upcoming neutrino experiments, such as the DUNE Near Detector, can probe previously unexplored parameter space for HNL masses up to $\mathcal{O}(1)~\mathrm{GeV}$ through their decay into pions, $μ^+μ^-$, and $e^+e^-$ final states at the detector. The resulting signals exhibit distinctive kinematics, allowing efficient discrimination from neutrino-induced Standard Model backgrounds. Our results demonstrate that the muon bremsstrahlung mechanism substantially extends the discovery potential of accelerator neutrino experiments for HNLs and other dark sector particles.