Abstract
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.
Highlights
As a prototypical hidden sector, we consider the compelling possibility of a spontaneously broken “dark” U(1)D gauge symmetry, mediated by a vector boson called the “dark photon”, ZD
If a dark Higgs mechanism is responsible for the spontaneous breaking of the U(1)D gauge symmetry, the dark Higgs boson will in general have a renormalizable coupling to the 125 GeV Standard Model (SM)-like Higgs, resulting in a mixing between the two physical scalar states
Dark sectors with a broken U(1)D gauge group that kinetically mixes with the SM hypercharge are well motivated and appear in a variety of new physics scenarios
Summary
Diagonalizes the gauge boson kinetic terms in eq (2.1) (the subscript ‘0’ in ZD,0 indicates that this is not yet a mass eigenstate). The singlet scalar can be produced via the process pp → ZD∗ → ZDs, which occurs at the same order of as the exotic Higgs decay h → ZZD All of these channels should be studied more comprehensively in the future, but are beyond the scope of this paper. A large fraction of ZD will decay inside the detector for 10−7 (see right panel of figure 2), but the large luminosity of hadron colliders means that even ∼ 10−10 could be detected by looking for two displaced ZD → decays
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