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

Abstract

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 through displaced-vertex signals at the CEPC, with a center-of-mass energy of $\sqrt{s}=91.2 $ GeV and an integrated luminosity of $\mathcal{L}=$ $100$ ab$^{-1}$. After comparing several possible single-production processes for the photophobic ALP, we focus on the dominant process $e^+ e^- \to Z \to a \gamma $, in which the ALP $a$ subsequently decays into a pair of charged leptons at a displaced vertex. Dedicated Monte Carlo simulations are performed for the $\mu^+ \mu^- \gamma$ and $\tau_h^+ \tau_h^- E\mkern-10.5 mu/_T \gamma$ signals. For the $\mu^+\mu^-\gamma$ signal, the CEPC is sensitive to the parameter region $g_{aWW} \in [1.27\times10^{-3},6.80\times10^{-1}]~\mathrm{TeV}^{-1}$ for $m_a \in [1,4]~\mathrm{GeV}$. For the $\tau_h^+\tau_h^- E\mkern-10.5 mu/_T \gamma$ signal, the accessible region is $g_{aWW} \in [7.00\times10^{-4},9.40\times10^{-3}]~\mathrm{TeV}^{-1}$ for $m_a \in [4,9]~\mathrm{GeV}$. These results demonstrate the substantial potential of the CEPC to explore light, long-lived ALPs through displaced-vertex signals, providing complementary coverage to existing searches at LEP and the LHC, as well as to the projected reach of the HL-LHC.