Dark Photons in the Radio Sky: I. Resonant Conversions in Halos

Abstract

Mixing between dark photons and visible photons leads to substantial anisotropies in the cosmic microwave background due to resonant conversions of visible photons into dark photons in baryonic matter found in dark matter halos. In this Letter, we forecast the sensitivity of the Square Kilometre Array (SKA) to this signal. We find that SKA could be the first experiment to discover dark photons with a mass between $10^{-13}$ and $5\times 10^{-12}$ eV and kinetic mixing parameter $ε$ as small as $\sim 10^{-8}$ by cross-correlating their data with a low-redshift galaxy survey, potentially improving on the sensitivity from a similar analysis using Planck data by a factor of 4 in $ε$. This improvement is largely due to an enhancement of the signal at low frequencies and the unique experimental advantages of radio telescopes such as small beam sizes.

Figures (3)

• Figure 1: A cartoon representation of the analysis pipeline. We begin with the theoretical prediction of the signal in the halo model. We then cross-correlate this predicted signal with a mock galaxy catalog to get a theoretical power spectrum. Next, we simulate astrophysical foregrounds to get 9 mock foreground maps at SKA frequencies, which we process with the ILC algorithm. Finally, this null-signal result is compared with the theory prediction using a Gaussian likelihood.
• Figure 2: The power spectra of several post-ILC maps compared to the theoretical signal. (Left): The red and blue lines are the auto-power spectra from performing the ILC procedure on real and mock Planck maps at the actual beams, respectively. The orange and purple lines are the auto-power spectra of our post-ILC fiducial mock radio and mock Planck maps, respectively, with a 0.5 beam. (Right): The cross-power spectra of our fiducial post-ILC map with our mock $\delta_g$ is compared to the theoretical dark photon signal. In both plots, the theoretical signal (in black) is for a dark photon with $m_{A'}=\qty{5.6e-13}{\eV}$. https://github.com/bakerem/dark_photons_radio_sky/tree/main/notebooks_for_paper/Cl_Comparison.ipynb
• Figure 3: Our forecasted sensitivity for a future radio search for dark photons. Shown are the sensitivities from modeling $\gamma \to A'$ conversions in dark matter halos and $\gamma \to A'$ conversions in the IGM during the EoR. The band around the EoR IGM limits shows the uncertainty in our inferred limits. In both cases, we show the sensitivities from considering auto-correlations and cross-correlations with the relevant galaxy survey. Also shown is the expected sensitivity of a 21-cm global signal experiment to $\gamma \to A'$ conversions and our rudimentary estimate of the auto-correlation signal from conversions in the late universe IGM. https://github.com/bakerem/dark_photons_radio_sky/tree/main/notebooks_for_paper/Limits.ipynb