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 ${\tau }_h^{+}{\tau }_h^{-}E{/}_T\gamma$ signals. For the ${\mu }^{+}{\mu }^{-}\gamma$ signal, the CEPC is sensitive to the parameter region $g_{aWW}\in [1.27\times 10^{-3},6.80\times 10^{-1}]{\mathrm{TeV}}^{-1}$ for $m_a\in [1,4]\mathrm{GeV}$. For the ${\tau }_h^{+}{\tau }_h^{-}E{/}_T\gamma$ signal, the accessible region is $g_{aWW}\in [7.00\times 10^{-4},9.40\times 10^{-3}]{\mathrm{TeV}}^{-1}$ 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.

Comments: 26 pages, 11 figures, 4 tables, to be published in Chinese Physics C