  High-energy colliders offer a unique sensitivity to dark photons, the
mediators of a broken dark U(1) gauge theory that kinetically mixes with the
Standard Model (SM) hypercharge. Dark photons can be detected in the exotic
decay of the 125 GeV Higgs boson, h -> Z Z_D -> 4l, and in Drell-Yan events, pp
-> Z_D -> ll. If the dark U(1) is broken by a hidden-sector Higgs mechanism,
then mixing between the dark and SM Higgs bosons also allows the exotic decay h
-> Z_D Z_D -> 4l. We show that the 14 TeV LHC and a 100 TeV proton-proton
collider provide powerful probes of both exotic Higgs decay channels. In the
case of kinetic mixing alone, direct Drell-Yan production offers the best
sensitivity to Z_D, and can probe epsilon >~ 9 x 10^(-4) (4 x 10^(-4)) at the
HL-LHC (100 TeV pp collider). The exotic Higgs decay h -> Z Z_D offers slightly
weaker sensitivity, but both measurements are necessary to distinguish the
kinetically mixed dark photon from other scenarios. If Higgs mixing is also
present, then the decay h -> Z_D Z_D can allow sensitivity to the Z_D for
epsilon >~ 10^(-9) - 10^(-6) (10^(-10) - 10^(-7)) for the mass range 2 m_mu <
m_(Z_D) < m_h/2 by searching for displaced dark photon decays. We also compare
the Z_D sensitivity at pp colliders to the indirect, but model-independent,
sensitivity of global fits to electroweak precision observables. We perform a
global electroweak fit of the dark photon model, substantially updating
previous work in the literature. Electroweak precision measurements at LEP,
Tevatron, and the LHC exclude epsilon as low as 3 x 10^(-2). Sensitivity can be
improved by up to a factor of ~2 with HL-LHC data, and an additional factor of
~4 with ILC/GigaZ data.
