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

TL;DR

It is commented 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.

Abstract

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.

Figures (2)

• Figure 1: Laboratory-based upper limits on the fractional electric charge $\epsilon = Q_\epsilon/e$ of a hypothetical millicharged fermion of mass $m_\epsilon$. The "Beam dump" limit has been derived in Ref. Davidson:1991si from a beam-dump search for new neutrino-like particles at SLAC Donnelly:1978tyRothenberg:1982. The "Orthopositronium" limit stems from a limit on the branching fraction of invisible orthopositronium decay Mitsui:1993ha. The "Lamb shift" limit comes from a recent comparison Davidson:2000hf of Lamb shift measurements Lundeen:1981Hagley:1994 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 Cameron:1993mr (see text).
• Figure 2: Laboratory-based upper limits on the fractional electric charge $\epsilon = Q_\epsilon/e$ of a hypothetical millicharged fermion of mass $m_\epsilon$ (same as in Fig. \ref{['fig:lablimits']}). The parameter values between the two lines labelled "PVLAS dichroism" correspond to the preferred 95 % confidence region if the PVLAS rotation is interpreted as orginating from pair production of millicharged fermions. The dashed limit labelled "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.