Evading Astrophysical Constraints on Axion-Like Particles

TL;DR

The paper addresses the tension between a light axion-like particle coupling to photons suggested by the PVLAS experiment and the stringent astrophysical and CAST bounds. It develops two routes to reconcile the data: (i) a trapping regime where φ interacts strongly with a paraphoton sector in stars, increasing opacity and preventing escape, and (ii) a suppression mechanism in which φ production in stellar interiors is cut by a momentum-dependent form factor arising from a composite preon structure. The analysis shows that the trapping scenario requires very small mixing and low-energy scales but runs into cosmological background constraints, while the suppression scenario can, in principle, accommodate the PVLAS value with a sub-eV preon mass, albeit with highly speculative low-energy new physics. Together, the results indicate that explaining PVLAS within conventional ALP frameworks would demand new, very low-energy physics, and that the CAST non-observation would not necessarily rule out PVLAS-like couplings if such mechanisms are viable.

Abstract

Stellar energy loss arguments lead to strong constraints on the coupling $φγγ$ of a light axion-like particle to two photons. Helioscopes, like CAST, are able to put competitive bounds. The PVLAS experiment has recently observed a rotation of the polarization of a laser propagating in a magnetic field that can be interpreted as the effect of a quite strong $φγγ$ coupling. We present scenarios where the astrophysical and CAST bounds can be evaded, and we show that the PVLAS result can be accomodated in one of the models, provided the new physics scale is at very low energies.