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

Possible extensions of the standard model of particle physics suggest the existence of particles with small, unquantized electric charge. Photon-initiated pair production of millicharged fermions in a magnetic field would manifest itself as a vacuum magnetic (VM) 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 previous laboratory searches. VM birefringence due to virtual pair production gives a slightly better constraint for masses between 0.1 and a few eV. We comment on the possibility that the VM dichroism observed by PVLAS arises from pair production of such millicharged fermions rather than from single production of axionlike particles. Such a scenario can be confirmed or firmly excluded by a search for invisible decays of orthopositronium with a branching-fraction sensitivity of about ${10}^{-9}$.

Figure 1

Laboratory-based upper limits on the fractional electric charge $ϵ=Q_{ϵ}/e$ of a hypothetical millicharged fermion of mass $m_{ϵ}$. The “beam-dump” limit has been derived in Ref. 14 from a beam-dump search for new neutrinolike particles at SLAC [49, 50]. The “orthopositronium” limit stems from a limit on the branching fraction of invisible orthopositronium decay [21]. The “Lamb-shift” limit comes from a recent comparison [20] of Lamb-shift measurements [22, 23] with QED predictions. The “BFRT dichroism-birefringence” limit arises from the upper limit on vacuum magnetic dichroism/birefringence placed by the laser polarization experiment BFRT [35] (see text).

Figure 2

Laboratory-based upper limits on the fractional electric charge $ϵ=Q_{ϵ}/e$ of a hypothetical millicharged fermion of mass $m_{ϵ}$ (same as in Fig. 1). The parameter values between the two lines labeled “PVLAS dichroism” correspond to the preferred 95% confidence region if the PVLAS rotation is interpreted as originating from pair production of millicharged fermions. The dashed limit labeled “orthopositronium (future)” corresponds to the projected 95% exclusion limit obtainable through a search for invisible orthopositronium decay with a sensitivity of ${10}^{-9}$ in the corresponding branching ratio.