Shining ALP Dark Radiation

TL;DR

This paper investigates how decaying axion-like particles (ALPs) from string scenarios can produce detectable signals through their decay to photons, allowing constraints on their properties using astrophysical observations.

Contribution

It provides a detailed analysis of non-thermally produced relativistic ALPs decaying into photons and explores the potential to probe their couplings at very high energy scales.

Findings

ALPs with mass > MeV and contributing to ΔN_eff > 0.001 can be constrained.

Observations of X-ray, gamma-ray, CMB, and BBN data can probe ALP-photon couplings at string or Planck scale.

Very small ALP-photon couplings are accessible through current astrophysical observations.

Abstract

String scenarios typically not only predict axion-like particles (ALPs) but also significant amounts of ALP dark radiation originating from the decay of the inflaton or a more general modulus. In this paper, we study the decay of such non-thermally produced relativistic (but massive) ALPs to photons. If the ALPs are sufficiently highly energetic, contribute to $\Delta N_{\rm eff} \gtrsim {\cal O}(0.001)$ and have a mass $m_a\gtrsim$ MeV we find that, using observations of X-, and $\gamma$-rays, the CMB and BBN, very small values of the ALP-photon coupling can be probed, corresponding to an origin of this coupling at the string (or even Planck) scale.