Cosmic-Ray Search for Fractionally Charged Particles

Abstract

A mountain-altitude search has been made in the cosmic-ray spectra for the hypothetical, fractionally charged particles known as quarks. These particles of charge $\frac{1}{3}e$ and $\frac{2}{3}e$ have been theorized as the fundamental particles underlying one possible interpretation of the $\mathrm{SU}\left(3\right)\mathrm{}$ classification schemes. A six-element, liquid-scintillator telescope, with the two central elements independent of the triggering requirements, was used in this search. The data were recorded in the form of differentially delayed pulses from the six photomultipliers. Using 90% confidence levels we have found upper limits to the vertical intensities of $\frac{1}{3}e$ and $\frac{2}{3}e$ quarks at 760 g/${\mathrm{cm}}^2$ atmospheric depth to be $I_Q\left(\frac{1}{3}\right)\le 8.7\times {10}^{-9\mathrm{}}$ ${\mathrm{cm}}^{-2\mathrm{}}$ ${\sec }^{-1\mathrm{}}$ ${\mathrm{sr}}^{-1\mathrm{}}$ and $I_Q\left(\frac{2}{3}\right)\le 1.8\times {10}^{-8\mathrm{}}$ ${\mathrm{cm}}^{-2\mathrm{}}$ ${\sec }^{-1\mathrm{}}$ ${\mathrm{sr}}^{-1\mathrm{}}$, respectively. The upper limits to the production cross sections have been estimated as a function of the masses of these hypothetical particles and assumed values for attenuation mean free paths. If one assumes that the quark-production cross sections are of the order of 0.01 mb and that the quark-removal cross sections are less than 15 mb/nucleon, then $M_Q\left(\frac{1}{3}\right)\ge 9$ BeV/${\mathit{c}}^2$ and $M_Q\left(\frac{2}{3}\right)\ge 7$ BeV/${\mathit{c}}^2$ are the lower limits obtained in this experiment.