  Dark photons are predicted by various new physics models, and are being
intensively studied in a variety of experiments. In the first part of this
paper, we obtain partial wave unitarity constraints on the dark photon
parameter space from the allowed $VV\rightarrow VV$ scattering processes in the
limit of large center-of-mass energy, where $V=W,Z$. In the second part of the
paper, searches are performed using the expected differential rates with a
realistic detector simulation including a comprehensive set of background
processes on dilepton and dilepton plus a photon events at the High Luminosity
LHC. In these searches, sensitive differential distributions are used in an
optimized way to determine the sensitivity to dark photon parameter space. It
is shown that remarkable sensitivity to the dark photon model is achieved and
kinetic mixing strength can be probed down to $(1.4-10)\times 10^{-4}$ for dark
photon mass between $15$ GeV to $2$ TeV. We also investigate the sensitivity of
a future muon collider suggested by the Muon Accelerator Program (MAP) to the
dark photon model at different center-of-mass energies. It is shown that a
future muon collider is able to reach a sensitivity to kinetic mixing at the
order of $10^{-4}$.
