Search for Millicharged Particles at SLAC

Particles with electric charge $q\ensuremath{\le}{10}^{\ensuremath{-}3}e$ and masses in the range $1--100\mathrm{MeV}{/c}^{2}$ are not excluded by present experiments. An experiment uniquely suited to the production and detection of such ``millicharged'' 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%-confidence upper limit on electric charge of $4.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}e$ for millicharged particles of mass $1\mathrm{MeV}{/c}^{2}$ and $5.8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}e$ for mass $100\mathrm{MeV}{/c}^{2}$.