Search for Fractionally Charged Particles Produced by Cosmic Rays

Measurements were made of the flux of particles with charges of one-third and two-thirds the charge of the electron, which reach sea level from the vertical with relativistic velocities. A set of six scintillation counters with an acceptance of 650 ${\mathrm{cm}}^2$ sr was directed so that the particles passed through the array. The apparatus was in operation for about 3500 h. Particles with fractional charge were identified through their characteristic energy loss in each of the six counters. The flux of particles with a charge of one-third was determined to be ($2.6_{-1.3\mathrm{}}^{}{}_{}{}^{+2.1\mathrm{}}$)×${10}^{-9\mathrm{}}$ ${\mathrm{cm}}^{-2\mathrm{}}$ ${\mathrm{sr}}^{-1\mathrm{}}$ ${\sec }^{-1\mathrm{}}$. The flux of particles with a charge of two-thirds was determined to be ($2.1_{-1.5\mathrm{}}^{}{}_{}{}^{+1.8\mathrm{}}$)×${10}^{-9\mathrm{}}$ ${\mathrm{cm}}^{-2\mathrm{}}$ ${\mathrm{sr}}^{-1\mathrm{}}$ ${\sec }^{-1\mathrm{}}$. The statistical probability that no anomalously charged particles were detected is about 5%.