Probing the axion-photon-dark photon interaction at future e + e − colliders

We study the interaction between photons, dark photons, and axions at future lepton colliders, focusing on single-photon events with missing energy as the experimental signature. We find that future facilities such as the International Linear Collider, Circular Electron-Positron Collider, and Future Circular Collider will be sensitive to the axion–photon–dark photon coupling down to the order of 10 − 4 GeV − 1 for dark photon masses around O ( 10 GeV ) , assuming that the axion is extremely light and escapes detection. We further show that longitudinal beam polarization at the International Linear Collider can enhance the signal significance by a factor of 4, providing the strongest projected reach in the model parameter space. Existing constraints from Large Electron-Positron II are analyzed for comparison. Furthermore, the mass of dark photon can be determined by measuring the sharp dropoff in the distribution of the recoil mass.