A search has been made for massive, long-lived, weakly interacting particles of fractional charge that could be produced by 300-GeV protons striking an aluminum target. The search was most sensitive to particles with a lifetime of about 100 μsec. These particles would have come to rest in the Caltech neutrino detector that was located halfway down the muon shield, and if they were massive enough and decayed into at least 5 GeV of visible energy in the calorimeter, they could have been detected. None were found. The 90%-confidence-level upper limit to the invariant cross section (times branching ratio $R$), $\mathrm{RE}\frac{d^3\sigma }{d{p}^3}$ [${\mathrm{cm}}^2$/${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^2$], is 4 × ${10}^{-39\mathrm{}}$ (5 × ${10}^{-38\mathrm{}}$) for charge $\left(\frac{2}{3}\right)e$ ($\left(\frac{1}{3}\right)e$).

• Received 19 January 1979

DOI:https://doi.org/10.1103/PhysRevD.20.82