Detecting interactions between dark matter and photons at high energy $e^{+}{e}^{-}$ colliders

We investigate the sensitivity to the effective operators describing interactions between dark matter particles and photons at future high energy $e^{+}{e}^{-}$ colliders via the $\gamma +\text{slashed}E$ channel. Such operators could be useful to interpret the potential gamma-ray line signature observed by the Fermi-LAT. We find that these operators can be further tested at $e^{+}{e}^{-}$ colliders by using either unpolarized or polarized beams. We also derive a general unitarity condition for $2\to n$ processes and apply it to the dark matter production process $e^{+}{e}^{-}\to \chi \chi \gamma$.