Sensitivity of the FCC-ee to axion-like particles at different center-of-mass energies
The sensitivity of the proposed FCC-ee collider to axion-like particles (ALPs) is investigated at all planned center-of-mass energies, focusing on the case where the ALP couples primarily to electroweak gauge bosons at leading order. We study the associated production of an ALP with a photon, with the ALP decaying in turn to two photons, yielding a three-photon final state. Expected 95% confidence level sensitivities to the ALP-photon coupling are computed for ALP masses between 5 and 320 GeV. In the benchmark model considered, the FCC-ee can probe the ALP-photon coupling down to a few $10^{-6}$ GeV$^{-1}$ at the Z pole and about $10^{-5}$ GeV$^{-1}$ at the WW, ZH, and $t\bar{t}$ running stages. The higher-energy running stages extend the mass reach beyond the Z-pole kinematic range, while the Z-pole run provides the best sensitivity at low masses. We also study how the sensitivity changes when the relative coupling strengths to the electroweak gauge fields are varied, showing that the three-photon channel can provide information complementary to photon-fusion probes of ALPs.