Illuminating dark photons with high-energy colliders

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 --> ZZ (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 greater than or similar to 9 x 10(-4) (4 x 10(-4)) at the HL-LHC (100 TeV pp collider). The exotic Higgs decay h --> ZZ (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 greater than or similar to 10(-9) - 10(-6) (10(-10) - 10(-7)) for the mass range 2m mu < mZ (D) < mh/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 similar to 2 with HL-LHC data, and an additional factor of similar to 4 with ILC/GigaZ data.