Searches for axion-like particles via γγ fusion at future e^+e^- colliders

Opportunities for searches for axion-like particles (ALPs) coupling to photons in e^+e^- collisions at the Future Circular Collider (FCC-ee) and International Linear Collider (ILC) are investigated. We perform a study of the photon-fusion production of ALPs decaying into two photons, e^+e^- γγ⟶e^+ a(γγ) e^-, over the light-by-light continuum background, for the planned FCC-ee and ILC center-of-mass energies and integrated luminosities. An analysis of the feasibility measurements is presented using parametrized simulations for two types of detectors. Upper limits at 95 σ(γγ→ a →γγ), and on the ALP-photon coupling are obtained over the m_a ≈ 0.1–1000 GeV ALP mass range, and compared to current and future collider searches. Production cross sections down to σ(γγ→ a →γγ) ≈ 1 fb (1 ab) will be probed at m_a≈ 1 (300) GeV, corresponding to constraints on the axion-photon coupling as low as g_aγγ≈ 2·10^-3 TeV^-1.