Polarized Light Propagating in a Magnetic Field as a Probe of Millicharged Fermions

Possible extensions of the standard model of elementary particle physics suggest the existence of particles with small, unquantized electric charge. Photon initiated pair production of millicharged fermions in an external magnetic field would manifest itself as a vacuum magnetic dichroism. We show that laser polarization experiments searching for this effect yield, in the mass range below 0.1 eV, much stronger constraints on millicharged fermions than previously considered laboratory searches. Vacuum magnetic birefringence originating from virtual pair production gives a slightly better constraint for masses between 0.1 eV and a few eV. We comment on the possibility that the vacuum magnetic dichroism observed by PVLAS arises from pair production of such millicharged fermions rather than from single production of axion-like particles. Such a scenario can be confirmed or firmly excluded by a search for invisible decays of orthopositronium with a sensitivity of about 10^(-9) in the corresponding branching fraction.