  The dark (or hidden) photon is a massive U(1) gauge boson theorized as a dark
force mediator. Dark photons can be detected with axion cavity haloscopes by
probing for a power excess caused by the dark photon's kinetic mixing with
Standard Model photons. Haloscope axion exclusion limits may therefore be
converted into competitive dark photon parameter limits via the calculation of
a corresponding dark photon to photon coupling factor. This calculation allows
for an improvement in sensitivity of around four orders of magnitude relative
to other dark photon exclusions and may be attained using existing data. We
present how one converts haloscope axion search limits and a summary of
relevant experimental parameter from published searches. Finally, we present
limits on the kinetic mixing coefficient between dark photons and the Standard
Model photons based on existing haloscope axion searches.
