Searching for dark photons from dark-scalar decays at CEPC and FCC-ee

TL;DR

This work investigates the potential to discover long-lived dark photons produced from decays of a dark scalar at future $e^+e^-$ Higgs-factory colliders CEPC and FCC-ee operating at $\sqrt{s}=240$ GeV. By modeling a dark-sector with a kinetically mixed dark photon ($\epsilon$) and a Higgs-portal mixed dark scalar ($\theta$), the authors study $e^+e^-\to Z\phi$ production followed by $\phi\to \gamma'\gamma'$, with $\gamma'$ subsequently decaying to SM fermions. They develop two displaced-vertex strategies within the inner tracker, compute detector acceptances via MC simulations, and project 95% CL sensitivities in the $(m_{\gamma'}, \epsilon)$ plane for representative $m_\phi$ values, showing sensitivity improvements of several orders of magnitude beyond current bounds for $m_\phi\sim10$–$100$ GeV and $m_{\gamma'}=2$ GeV–$m_\phi/2$. The results highlight the capability of CEPC and FCC-ee to probe dark-photon parameter space inaccessible to existing experiments, while acknowledging optimistic detector assumptions and the need for more realistic detector-level modeling in future work. Overall, the study demonstrates a promising pathway to testing dark-sector portals with displaced-lepton signatures at next-generation lepton colliders.

Abstract

We investigate the sensitivity of proposed CEPC and FCC-ee with a center-of-mass energy of 240 GeV to long-lived dark photons heavier than 2 GeV that are pair-produced via the prompt decays of a light scalar mixed with the Standard-Model Higgs boson. We compute the production and decay rates of both the light scalar and the dark photon, and develop two search strategies targeting displaced vertices within the inner tracker of the main detectors. Using Monte Carlo simulations, we evaluate the signal acceptance and projected sensitivity for each strategy. Our results show that, for the scalar-Higgs mixing angle set at $10^{-2}$ just below the current upper limit, the proposed searches at CEPC and FCC-ee can probe dark-photon kinetic-mixing parameter several orders of magnitude below existing bounds, for dark photons lighter than half the dark-scalar mass.

Figures (6)

• Figure 1: The production cross section of $\phi$ via the process $e^+ e^- \to Z \,\phi$ rescaled with $1/\theta^2$ as a function of the dark-scalar mass $m_\phi$, computed with MadGraph5Alwall:2014hca, at the COM energy $\sqrt{s} = 240$ GeV.
• Figure 2: The proper decay length $c\tau_{\gamma'}$ of the dark photon, rescaled with $\epsilon^2$, as a function of the dark-photon mass.
• Figure 3: The decay branching ratios of the dark photon into $e^+e^-$ (red solid) and $\mu^+\mu-$ (blue dotted), respectively, as functions of the dark-photon mass. Note that the two curves overlap for the whole range of $m_{\gamma'}$ shown here. The kinks at $m_{\gamma'}\sim 2.5$ GeV, 3.5 GeV, and 8.3 GeV, arise from the opening of the dark-photon decay channels into a pair of charm quarks, $\tau$-leptons, and bottom quarks, respectively.
• Figure 4: The Feynman diagrams for production (upper plot) and decay (lower plot) of the dark scalar.
• Figure 5: A profile sketch of the inner tracker at CEPC and FCC-ee, extracted from Ref. Cheung:2019qdr.