  Hidden U(1) gauge symmetries are common to many extensions of the Standard
Model proposed to explain dark matter. The hidden gauge vector bosons of such
extensions may mix kinetically with Standard Model photons, providing a means
for electromagnetic power to pass through conducting barriers. The ADMX
detector was used to search for hidden vector bosons originating in an emitter
cavity driven with microwave power. We exclude hidden vector bosons with
kinetic couplings {\chi} > 3.48x10-8 for masses less than 3 {\mu}eV. This limit
represents an improvement of more than two orders of magnitude in sensitivity
relative to previous cavity experiments.
