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 impose 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 accommodated in one of the models, provided that the new physics scale is at very low energies. © 2005 IOP Publishing Ltd.
- Magnetic fields