Search for Millicharged Particles at SLAC

Particles with electric charge q{le}10{sup {minus}3}e and masses in the range 1{endash}100 MeV/c{sup 2} are not excluded by present experiments. An experiment uniquely suited to the production and detection of such {open_quotes}millicharged{close_quotes} 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{percent}-confidence upper limit on electric charge of 4.1{times}10{sup {minus}5}e for millicharged particles of mass 1 MeV/c{sup 2} and 5.8{times}10{sup {minus}4}e for mass 100 MeV/c{sup 2} . {copyright} {ital 1998} {ital The American Physical Society }