Search for Fractionally Charged Particles in Cosmic Rays Near Sea Level

An array of spark chambers and scintillation-counter trays has been used to search for fractionally charged particles in cosmic rays near sea level. No acceptable events have been found with energy losses by ionization between 0.04 and 0.7 that of singly charged minimum-ionizing particles. This experiment sets new upper limits for the fluxes of fractionally charged particles in cosmic rays, namely, 1.7×${10}^{-10\mathrm{}}$ and 3.4×${10}^{-10\mathrm{}}$ ${\mathrm{cm}}^{-2\mathrm{}}$ ${\mathrm{sr}}^{-1\mathrm{}}$ ${\sec }^{-1\mathrm{}}$ (90% confidence) for minimum-ionizing particles with charges $\frac{1}{3}$ and $\frac{2}{3}$, respectively.