Revisiting the Dark Photon Explanation of the Muon Anomalous Magnetic
  Moment
  A massive $U(1)^{\prime}$ gauge boson known as a "dark photon" or
$A^{\prime}$, has long been proposed as a potential explanation for the
discrepancy observed between the experimental measurement and theoretical
determination of the anomalous magnetic moment of the muon, ($g_{\mu} - 2$)
anomaly. Recently, experimental results have excluded this possibility for a
dark photon exhibiting exclusively visible or invisible decays. In this work,
we revisit this idea and consider a model where $A^{\prime}$ couples
inelastically to dark matter and an excited dark sector state, leading to a
more exotic decay topology we refer to as a semi-visible decay. We show that
for large mass splittings between the dark sector states this decay mode is
enhanced, weakening the previous invisibly decaying dark photon bounds. As a
consequence, $A^{\prime}$ resolves the $g_{\mu} - 2$ anomaly in a region of
parameter space the thermal dark matter component of the Universe is readily
explained. Interestingly, it is possible that the semi-visible events we
discuss may have been vetoed by experiments searching for invisible dark photon
decays. A re-analysis of the data and future searches may be crucial in
uncovering this exotic decay mode or closing the window on the dark photon
explanation of the $g_{\mu} - 2$ anomaly.
