Discovery Prospects for a Leptophilic Gauge Boson $Z_\ell$ at CEPC and ILC

Abstract

We investigate the discovery prospects of a leptophilic gauge boson Z_l at future $e^+e^-$ colliders, focusing on a comparative study of the Circular Electron-Positron Collider (CEPC) and the International Linear Collider (ILC). This state can arise from an additional U(1)'_l gauge symmetry under which quarks are neutral and all leptons have a universal charge, motivated by neutrino oscillations and physics beyond the Standard Model. As a clean benchmark we study $e^+e^- \to \mu^+\mu^-$ including realistic initial state radiation (ISR) and beamstrahlung. We find that CEPC at sqrt(s)=240 GeV can probe couplings down to $g_l \sim 10^{-3}$ for $Z_l$ masses up to about 220 GeV, while the ILC extends the sensitivity to heavier states in the multi-hundred GeV range through its higher sqrt(s) stages. These results highlight the complementarity of circular and linear colliders in testing purely leptophilic interactions. Published in Chinese Physics C 50 (2026) 033104. DOI: 10.1088/1674-1137/ae1de4