Search for Quarks

Fractionally charged particles (quarks) have been looked for in cosmic rays at sea level. The detection system consists of twelve layers of scintillation counters and a streamer chamber with an over-all acceptance of 0.20×${10}^4$ ${\mathrm{cm}}^2$ sr. Particles of charges $\frac{1}{3}e$ and $\frac{2}{3}e$ were identified by their characteristic energy losses in each of the twelve counters as well as by a single straight track in the streamer chamber. Coincidences of many low-energy photons are responsible for the background events, as is shown by features of the particle tracks in the streamer chamber and by the fluctuation of the flight time in the counter array. In selecting quarks from the backgrounds, the observed events were checked by: (i) a test for correlation among the individual pulse heights; (ii) a test for the arrival time correlation; (iii) the requirement of a single straight track in the streamer chamber which could penetrate the whole counter system if extrapolated. During a total of 3500 h of operation, there was one event for $\frac{1}{3}e$ charge and none for $\frac{2}{3}e$ charge which satisfied the above requirements. This one event occurred when the chamber was not yet in operation. The upper limits obtained on the flux of relativistic quarks in cosmic rays at sea level are 0.5×${10}^{-10\mathrm{}}$ ${\mathrm{cm}}^{-2\mathrm{}}$ ${\mathrm{sr}}^{-1\mathrm{}}$ ${\sec }^{-1\mathrm{}}$ for $\frac{1}{3}e$, and 7.5×${10}^{-10\mathrm{}}$ ${\mathrm{cm}}^{-2\mathrm{}}$ ${\mathrm{sr}}^{-1\mathrm{}}$ ${\sec }^{-1\mathrm{}}$ for $\frac{2}{3}e$.