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{}=2{ε}e where {ε}{}1. We put constraints on the mass (mₛ) and effective charge (i.e., {ε}) of such particles by examining their possible production and detection in high-statistics proton beam-dump experiments. We find, for example, that {ε}{}2×{}10^-2 for 1{}mₛ{}200 MeV.