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Circularly Polarized Gamma Rays in Effective Dark Matter Theory

We study the loop-induced circularly polarized gamma rays from dark matter annihilation using the effective dark matter theory approach. Both neutral scalar and fermionic dark matter annihilating into monochromatic diphoton and $Z$-photon final states are considered. To generate the circular polarization asymmetry, $P$ and $CP$ symmetries must be violated in the couplings between dark matter and Standard Model fermions inside the loop with non-vanishing Cutkosky cut. The asymmetry can be sizable especially for $Z$-photon final state for which asymmetry of nearly $90\%$ can be reached. We discuss the prospect for detecting the circular polarization asymmetry of the gamma-ray flux from dark matter annihilation in the Galactic Center in future gamma-ray polarimetry experiments.