The photomagneton is a theoretical treatment of the unitary group in quantum field theory and quantum chemistry that effectively describes the experimentally observed inverse Faraday effect.
When circularly polarized light travels through a plasma, the angular momentum associated to the circular motion of the photons induces an angular momentum in the electrons of the plasma.
For instance, if the so-called ghost field does not contribute to the free electromagnetic energy density in the plasma, then the electron must couple to something like a complex electric field.
However, if the field induces a finite magnetic field in the absence of matter, then the implication may be a finite photon rest mass.
A. Hasanein and M. Evans, The Photomagneton and Quantum Field Theory: Vol.