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Particles with electric charge q ≤ 10−3 e and masses in the range 1-100 MeV/c 2 are not excluded by present experiments. An experiment uniquely suited to the production and detection of such "millicharged" particles has been carried out at SLAC. This experiment is sensitive to the infrequent excitation and ionization of matter expected from the passage of such a particle. Analysis of the data rules out a region of mass and charge, establishing, for example, a 95%-confidence upper limit on electric charge of 4.1×10−5 e for millicharged particles of mass 1 MeV/c 2 and 5.8×10 −4 e for mass 100 MeV/c 2 .PACS numbers: 14.80.-j, 95.35.+dThe quantization of electric charge is an empirically well-supported idea. Of the numerous searches for fractional charge carried out thus far, none has provided conclusive evidence for charge non-quantization. The current bounds on the proton-electron charge difference [1] and the neutron charge [2], of order 10−21 e, lend strong support to the notion that charge quantization is a fundamental principle. However, the Standard Model with three generations of quarks and leptons does not impose charge quantization. In order to enforce quantization of charge, physics beyond the Standard Model is necessary [3]. This observation has stimulated inquiry into mechanisms whereby charge quantization (and perhaps even charge conservation) might be violated [4]. Particles with small fractional charge (q < ∼ 10 −2 e) appear as a natural consequence of many of these mechanisms. There has been interest in the possibility of a small, nonzero electric charge for the neutrino [5], and the possibility that particles with small fractional charge account for a portion of the dark matter in the universe [6]. Additionally, a noteworthy model has been proposed wherein certain particles could exhibit apparent fractional charge without violating charge quantization [7]. Several authors have investigated constraints, imposed by laboratory experiments and by astrophysical and cosmological arguments, on the existence of (free) fractionally charged particles [8]. They point out that there remains a large domain in mass and charge (10