Illuminating Dark Photons with High-Energy Colliders

TL;DR

This work analyzes a kinetically mixed dark U(1)_D and, optionally, a Higgs portal, to study dark photons Z_D across collider and precision observable channels. It develops the theoretical framework for gauge and Higgs sectors, and computes decay and production rates for h → Z Z_D, h → Z_D Z_D, and pp → Z_D → ℓℓ, juxtaposed with electroweak precision constraints. The study provides projected sensitivities for the 14 TeV LHC and a future 100 TeV collider, highlighting that Drell-Yan production and exotic Higgs decays offer complementary probes—especially when Higgs mixing enables h → Z_D Z_D and possibly displaced Z_D decays. It demonstrates that future detectors and collider designs can sharply extend bounds on the kinetic mixing ε, potentially down to 10^-9–10^-6 in certain scenarios, thereby probing vast regions of hidden-sector parameter space.

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.

Figures (14)

• Figure 1: Exotic Higgs decays to four leptons induced by intermediate dark photons in the higgsed dark $U(1)$ model. Left:$h\to Z_D Z^{(*)} \to 4\ell$ via the hypercharge portal. Right:$h \to Z_D Z_D \to 4\ell$ via the Higgs portal.
• Figure 2: Left: Leptonic branching fraction of $Z_D$. Right: Decay length of $Z_D$ for different $\epsilon$. The dashed lines indicate boundaries between qualitatively different experimental regimes: prompt decay for $c \tau \lesssim 1 \mu$m and likely escape from an ATLAS-size detector for $c \tau \gtrsim 20$m.
• Figure 3: Br$(h\to Z_D Z^* \to 4\ell)$(top) and Br$(h\to Z_D Z_D \to 4\ell)$(bottom) for different values of $\epsilon$ and $\kappa'$.
• Figure 4: Present bound (purple shaded region) on the kinetic mixing coefficient $\epsilon$ from the fit to electroweak precision observables. Future projected reach at the 14 TeV LHC with 300 fb$^{-1}$ and 3000 fb$^{-1}$ of data, and at the ILC/GigaZ are shown by the dashed, dot-dashed, and dotted lines, respectively. Purple and green lines, respectively, represent the bounds obtained by keeping the central values of the measurements as they are now, or with central values adjusted to the values predicted by the SM best fit. For the ILC/GigaZ bound, we also assume the 14 TeV LHC (3000 fb$^{-1}$ data) precision measurements of $m_h$ and $m_t$. The HL-LHC and ILC/GigaZ projections also include expected improvements in the measurement of $\Delta\alpha^{(5)}_{\rm had}$ from VEPP-2000/Babar data.
• Figure 5: Predicted Higgs $p_T$ spectra in gluon fusion production, as calculated by matched MadGraph 5 + Pythia 6 (red) and HqT at LO+NLL (dashed blue) and NLO+NNLL (solid blue), for $\sqrt{s} =$ 100 TeV (left) and 14 TeV (right).