Shining ALP dark radiation

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 Delta N-eff greater than or similar to O(0.001), and have a mass m(a) greater than or similar to 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.

Automated summary

This paper studies the decay of nonthermally produced relativistic ALPs to photons, which could lead to probing very small values of the ALP-photon coupling through observations of x- and gamma-rays, the CMB and BBN if certain conditions are met.