A search for millicharged particles at SLAC

Particles with electric charge q/e less than or equal to 10(exp -3) and masses in the range 1-1000 MeV/c(sup 2) are not excluded by present experiments or by astrophysical or cosmological arguments. Such millicharged particles may be a form of shadow matter that couples to ordinary matter by virtue of shadow photon/photon mixing. This paper describes a search for millicharged particles in a dedicated beam-dump experiment at SLAC. A preliminary analysis of part of this data excludes particles of mass (approximately) 1 MeV/c(sup 2) with q/e greater than 6 x 10(exp -5).