Dark Photons in the Radio Sky: II. Resonant Conversions in the Intergalactic Medium

This is the second part in a pair of papers forecasting the sensitivity of the Square Kilometre Array (SKA) to dark photons, a highly motivated, simple extension of the Standard Model. Through a kinetic mixing term, visible photons from the cosmic microwave background can resonantly convert into dark photons, generating new temperature anisotropies in the sky. In this work, we detail the entire analysis pipeline that we use to compute SKA’s sensitivity, focusing on resonant conversions that occur in the intergalactic medium. We also discuss the sensitivity of 21-cm experiments to dark photons. Our results show that both SKA in combination with galaxy surveys and 21-cm experiments could discover dark photons with masses between $5 \times 10^{-15}$ and $5 \times 10^{-12}$ eV, and kinetic mixing parameter $\epsilon$ as low as $10^{-8}$.

arXiv:2511.09637