  A dark Z is a massive Abelian gauge boson which is coupled to the Standard
Model through both kinetic and mass mixing with the electroweak sector. We
study the phenomenology of the dark Z at an energy-frontier electron-positron
collider, such as the proposed International Linear Collider (ILC). We show
that precision electroweak constraints and the current bounds from the hadron
colliders allow a dark Z that is kinematically accessible at the ILC with 250
GeV or 500 GeV center-of-mass energy. Further, the reach of the ILC searches
for a dark Z significantly exceeds the expected reach of the high luminosity
LHC. If a signal consistent with a dark Z is discovered, it would motivate a
dedicated run of the ILC at the center-of-mass energy matching the dark Z mass.
We demonstrate that a short one month run at design luminosity could measure
the dark Z chiral couplings to fermions with percent precision. This
measurement can be used to discriminate between competing theoretical models of
the resonance: for example, the dark Z can be distinguished from a dark photon
with purely kinetic mixing.
