Probing Long-Lived Photophobic Axion-Like Particles via Prompt Leptons and Mono-$\gamma$ at FCC-ee and CEPC

Abstract

We investigate the potential to probe axion-like particles (ALPs) under the photophobic scenario at the FCC-ee and CEPC at the Z-pole, with $\sqrt{s} = 91.2$ GeV. The signal process is $$ e^+e^- \to Z \to \gamma a,\quad a \to \ell^+ \ell^-, $$ where we consider final states with two prompt leptons and one photon, or only one photon (Mono-$\gamma$). We estimate the sensitivity to the ALP mass $m_a$ and associated energy scale $\Lambda$ for $a\to\mu^+\mu^-$ and $a\to\tau^+\tau^-$ (with leptonic decays of the $\tau$) by use of a XGBoost classifier. For an integrated luminosity of 150 ab$^{-1}$ at the Z-pole, the combined leptonic channel can probe the ALP scale $\Lambda$ between $10$ to $700$~TeV, depending on the ALP mass. The Mono-$\gamma$ signal offers a complementary probe, reaching $\Lambda$ up to $2000$~TeV for masses below $20$~GeV.