Production and Detection
of Millicharged Particles


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 [hep-ph/0607118]. 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. Possibly, 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.

In [hep-ph/0608238], we have investigated Schwinger pair production of millicharged fermions in the strong electric field of cavities used for particle accelerators. Even without a direct detection mechanism at hand, millicharged particles, if they exist, contribute to the energy loss of the cavity and thus leave an imprint on the cavity's quality factor. Already conservative estimates substantially constrain the electric charge of these hypothetical particles; the resulting bounds are competitive with the currently best laboratory bounds which arise from experiments based on polarized laser light propagating in a magnetic field. We have proposed an experimental setup for measuring the electric current comprised of the millicharged particles produced in the cavity.

Such millicharges arise generically from kinetic-mixing in theories containing at least two U(1) gauge factors. In [hep-ph/0608248], we have pointed out that the required multiple U(1) factors, the size of kinetic-mixing, and suitable matter representations to explain the PVLAS data occur very naturally in the context of realistic extensions of the Standard Model (SM) of elementary particle physics based on string theory.


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Last modified: August 22, 2016
Andreas Ringwald