  Fixed-target proton-beam experiments produce a multitude of charged pions
that rescatter in the beam dump. These charged pion scattering events can be an
additional irreducible source of exotic particles which couple to photons or
hadrons. We analyze the sensitivity of the DUNE Near Detector complex to
millicharged particles (MCPs) and heavy axion-like particles (ALPs) with
low-energy couplings to gluons. Using the framework of chiral perturbation
theory, we demonstrate regimes of parameter space where the charged pion
production channel dominates over previously-considered production mechanisms
for both MCPs and ALPs, thereby improving the sensitivity of DUNE to these new
particles compared to previous studies.
