  The production and subsequent re-scattering of secondary pions produced in
proton beam dumps provides additional opportunities for the production of light
new particles like dark photons. This new mechanism has been overlooked in the
past but can extend the mass reach of the SpinQuest experiment and its proposed
DarkQuest upgrade. We use chiral perturbation theory to calculate the
production of kinetically mixed dark photons through bremsstrahlung off
secondary charged pions. We find that the reach of SpinQuest/DarkQuest can be
pushed further into the multi-GeV mass range compared to estimates based only
on primary dark photon production through meson decay or proton bremsstrahlung.
Our analysis can be regarded as the first of several steps to include secondary
pion contributions. In an upcoming analysis we will extend our calculation into
the high-momentum-transfer regime through the use of pion PDFs and including
hadronic resonances, which will further increase the estimated mass reach.
