Search for Quarks Produced with Large Transverse Momentum in 400-GeV Proton-Nucleus Collisions

We have searched for charge-$\frac{1}{3}$ and charge-$\frac{2}{3}$ particles produced with large $p_{\perp }$ in 400-GeV proton-Cu collisions. At $p_{\perp }=6.15\mathrm{}$ GeV/c, 4.9 × ${10}^4$ positive and 1.5 × ${10}^4$ negative unit-charge particles provided the normalization; no fractionally charged particles were found. The 90%-confidence-level upper limits for the invariant production cross sections per nucleon for positive and negative charge-$\frac{1}{3}$ particles are 5.1 × ${10}^{-39\mathrm{}}$ and 8.8 × ${10}^{-39\mathrm{}}$ ${\mathrm{cm}}^2$ Ge${\mathrm{V}}^{-2\mathrm{}}$, respectively; for charge-$\frac{2}{3}$ particles the respective limits are 1.3 × ${10}^{-39\mathrm{}}$ and 2.2 × ${10}^{-39\mathrm{}}$ ${\mathrm{cm}}^2$ Ge${\mathrm{V}}^{-2\mathrm{}}$.