String scenarios typically not only predict axionlike 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 nonthermally produced relativistic (but massive) ALPs to photons. If the ALPs are sufficiently highly energetic, contribute to $\mathrm{\Delta }{N}_{\mathrm{eff}}\gtrsim \mathcal{O}(0.001)$, and have a mass $m_a\gtrsim \mathrm{MeV}$ 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.

• Received 1 December 2021
• Accepted 13 May 2022

DOI:https://doi.org/10.1103/PhysRevD.105.115003

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Published by the American Physical Society