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 $$e^{+}{e}^{-}\to Z\to \gamma a,a\to {\ell }^{+}{\ell }^{-},$$ 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.