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We consider experimental limits on particles of small (not necessarily rational) electric charge. Such particles are possible within the standard model, and may be a natural consequence of extensions of the standard model incorporating an extra U(1) gauge group associated with a "mirror universe" sector. For both these cases we examine the limits from low-energy quantum electro-dynamics corrections, direct accelerator searches, stellar astrophysics, constraints on big-bang nucleosynthesis, and relic cosmological densities. The combined results exclude significant regions of the charge-mass parameter plane.
Davidson et al. (Mon,) studied this question.