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‖=2εe where ε≪1. We put constraints on the mass ($m_s$) 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×${10}^{\mathrm{-}2}$ for 1≲$m_s$≲200 MeV.