Search for fractionally charged particles in cosmic rays at large zenith angles

We have performed a single-particle search for fractional charge in cosmic rays having residual ranges at sea level >250 g/${\mathrm{cm}}^2$ concrete and zenith angles between 45° and 90°. The detector is a hodoscope of 24 layers of plastic scintillator, eight layers of multiwire proportional chambers, and two layers of lucite Cherenkov counters. The acceptance of the instrument is 4.0×${10}^3$ ${\mathrm{cm}}^2$sr. An analysis of 3.5×${10}^6$ triggers during a running time of 8×${10}^5$ sec yields no particles with charge $Q=\frac{1}{3}$ or $Q=\frac{2}{3}$ and velocities greater than $\approx 0.1c$. We deduce an upper limit on the flux of fractionally charged particles of 8.5×${10}^{-10\mathrm{}}$ ${\left({\mathrm{cm}}^2\mathrm{s}\mathrm{r}\mathrm{s}\mathrm{e}\mathrm{c}\right)}^{-1\mathrm{}}$ for relativistic $Q=\frac{1}{3}$ and 7.6×${10}^{-10\mathrm{}}$ ${\left({\mathrm{cm}}^2\mathrm{s}\mathrm{r}\mathrm{s}\mathrm{e}\mathrm{c}\right)}^{-1\mathrm{}}$ for $Q=\frac{2}{3}$.