A closer look at dark photon explanations of the excess radio background

TL;DR

This paper examines a dark photon-based model explaining the excess radio background, analyzing its spectral features, deviations from a power law, and potential tests with future observations like PIXIE.

Contribution

It provides a detailed analysis of the dark photon model's spectral predictions, highlighting deviations from a simple power law and proposing observational tests.

Findings

Deviations from power law at frequencies below 100 MHz and above 1 GHz.

Improved fit to radio background data with detailed spectral effects.

Potential detectability of signatures with future CMB spectral distortion experiments.

Abstract

The observed excess radio background has remained a puzzle for over a decade. A recent new physics solution involves dark matter that decays into dark photons in the presence of a thermal dark photon background. The produced non-thermal dark photon spectrum then converts into standard photons around the reionization era, yielding an approximate power-law radio excess with brightness temperature $T(\nu)\simeq \nu^{-2.5}$ over a wide range of frequencies, $\nu$. This simple power-law model comes intriguingly close to the current data, even if several ingredients are required to make it work. In this paper, we investigate some of the details of this model, showcasing the importance of individual effects. In particular, significant deviation from a power law are present at $\nu\lesssim 100\,{\rm MHz}$ and $\nu\gtrsim 1\,{\rm GHz}$. These effects result in improving the fit to data compared to a power-law spectrum, and may become testable in future observations. We also highlight independent signatures that can be tested with future CMB spectral distortion experiments such as {\it PIXIE}.