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. (C) 2019 The Authors. Published by Elsevier B.V.