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 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 $e^+ e^- \to Z \to a γ$, with the ALP $a$ subsequently decaying into a pair of displaced charged leptons. Dedicated Monte Carlo simulations are performed for the $μ^+ μ^- γ$ and $τ_h^+ τ_h^- E\mkern-10.5 mu/_T γ$ signals. For the $μ^+μ^-γ$ 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 $τ_h^+τ_h^-E\mkern-10.5 mu/_Tγ$ signal, the accessible region is $g_{aWW} \in [7.00\times10^{-4},9.40\times10^{-3}]~\mathrm{TeV}^{-1}$ with $m_a \in [4,9]~\mathrm{GeV}$. 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.

Figures (9)

• Figure 1: The branching ratios of different photophobic ALP decay modes as functions of its mass $m_a$.
• Figure 2: The proper decay length of the ALP as functions of its mass $m_a$ for several representative values of the coupling $g_{aWW}$.
• Figure 3: The Feynman diagrams for the single-production processes of the photophobic ALP, $e^+ e^- \to a \gamma$ (a), $a \nu \bar{\nu}$ (b,c), $a e^+ e^-$ (d,e) and $a jj$ (f).
• Figure 4: The production cross sections of the processes $e^+ e^- \to a \gamma$ (a), $a \nu \bar{\nu}$, $a e^+ e^-$ and $a jj$ (b) as functions of the ALP mass $m_a$ for different values of the coupling $g_{aWW}$ at the $91.2$ GeV CEPC.
• Figure 5: The production cross sections of the $\mu^+ \mu^- \gamma$ signal as functions of the ALP mass $m_a$ for several representative values of the coupling $g_{aWW}$ at the $91.2$ GeV CEPC.