Prospects for probing light photophobic axion-like particles via displaced vertex signals at the CEPC

In recent years, long-lived particles (LLPs) have attracted increasing attention in searches for physics beyond the Standard Model (SM). In this paper, we investigate the discovery prospects for light long-lived ALPs predicted by the photophobic ALP scenario via displaced vertex signals at the CEPC with the center-of-mass energy $\sqrt{s}=91.2$ GeV and integrated luminosity $\mathcal{L}=$ $100$ ab${}^{-1}$. After comparing several possible single-production processes of the photophobic ALP, we focus on the dominant process $e^{+}{e}^{-}\to Z\to a\gamma$, with the ALP $a$ subsequently decaying into a pair of displaced charged leptons. Dedicated Monte Carlo simulations are performed for the ${\mu }^{+}{\mu }^{-}\gamma$ and ${\tau }_h^{+}{\tau }_h^{-}E{\mathrm{/}}_T\gamma$ signals. For the ${\mu }^{+}{\mu }^{-}\gamma$ signal, the CEPC is sensitive to the parameter region $g_{aWW}\in [1.27\times {10}^{-3},6.80\times {10}^{-1}]{\mathrm{T}\mathrm{e}\mathrm{V}}^{-1}$ for $m_a\in [1,4]\mathrm{G}\mathrm{e}\mathrm{V}$. For the ${\tau }_h^{+}{\tau }_h^{-}E{\mathrm{/}}_T\gamma$ signal, the accessible region is $g_{aWW}\in [7.00\times {10}^{-4},9.40\times {10}^{-3}]{\mathrm{T}\mathrm{e}\mathrm{V}}^{-1}$ with $m_a\in [4,9]\mathrm{G}\mathrm{e}\mathrm{V}$. These results demonstrate the strong potential of the CEPC to explore light long-lived ALPs via displaced vertex signals, providing complementary coverage to existing searches at the LEP and LHC, as well as to the projected reach of the HL-LHC.