Effective field theory approach to light propagation in an external magnetic field

The recent PVLAS experiment observed rotation of polarization and ellipticity when a linearly polarized laser beam passes through a transverse magnetic field. The phenomenon cannot be explained in conventional QED. We attempt to accommodate the result by employing an effective theory for the electromagnetic field alone. No new particles with a mass of order the laser frequency or below are assumed. To quartic terms in the field strength, a parity-violating term appears besides the two ordinary terms. The rotation of polarization and ellipticity are computed for parity-asymmetric and -symmetric experimental set-ups. While rotation occurs in an ideal asymmetric case and has the same magnitude as ellipticity, it disappears in a symmetric set-up like PVLAS. This would mean that we have to appeal to some low-mass new particles with nontrivial interactions with photons to understand the PVLAS result. (orig.)