Sommerfeld Enhancement from Background Force and the Galactic Center GeV Excess

We study the impact of background-induced forces on dark matter (DM) annihilation and their implications for indirect detection. In the presence of a finite number density of background particles, loop-level interactions can generate an effective force that is significantly enhanced relative to the vacuum case. We construct a two-component DM model in which the dominant component is a fermionic particle $χ$ and the subdominant component is an ultralight pseudoscalar particle $ϕ$. The annihilation of $χ$ proceeds through the p-wave channel and produces gamma-ray emission. The finite density of $ϕ$ particles induces a background-enhanced force between $χ$ particles, leading to a sizable Sommerfeld enhancement of the annihilation. We show that a viable region of parameter space in this model can account for the gamma-ray excess observed in the Galactic Center using Fermi-LAT data. The background-induced force substantially amplifies the Sommerfeld enhancement and thus enlarges the parameter space capable of explaining the excess, highlighting the importance of background effects in astrophysical environments.

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