  We investigate the landscape of constraints on MeV-GeV scale, hidden U(1)
forces with nonzero axial-vector couplings to Standard Model fermions. While
the purely vector-coupled dark photon, which may arise from kinetic mixing, is
a well-motivated scenario, several MeV-scale anomalies motivate a theory with
axial couplings which can be UV-completed consistent with Standard Model gauge
invariance. Moreover, existing constraints on dark photons depend on products
of various combinations of axial and vector couplings, making it difficult to
isolate the effects of axial couplings for particular flavors of SM fermions.
We present a representative renormalizable, UV-complete model of a dark photon
with adjustable axial and vector couplings, discuss its general features, and
show how some UV constraints may be relaxed in a model with nonrenormalizable
Yukawa couplings at the expense of fine-tuning. We survey the existing
parameter space and the projected reach of planned experiments, briefly
commenting on the relevance of the allowed parameter space to low-energy
anomalies in pi^0 and 8-Be* decay.
