Limits on millicharged matter from beam-dump experiments

It has been proposed by Goldberg and Hall that the dark matter in galactic halos may be a type of shadow matter which interacts electromagnetically with ordinary matter (via a mixing effect) with effective charge Q=2e where 1. We put constraints on the mass (ms) and effective charge (i.e., ) of such particles by examining their possible production and detection in high-statistics proton beam-dump experiments. We find, for example, that 2×10-2 for 1ms200 MeV.