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The data on hadron--nucleus collisions at 400--700 GeV, collected by theSELEX experiment at Fermilab, are analyzed for formation of the exoticpentaquark baryon $\Theta^+(1540)$. A narrow enhancement near 1539 MeV isobserved in the mass spectrum of the $pK^0_S$ system emitted at small $x_F$from hadron collisions with copper nuclei. The statistical significance of thepeak is near 9 standard deviations. Fitted width of the observed $pK^0_S$resonance is consistent with being entirely due to experimental resolution, andits intrinsic width is restricted to $\Gamma < 3$ MeV at 90\% CL. The datafavor positive rather than negative strangeness for the $pK^0_S$ resonanceobserved in $h$Cu collisions. At the same time, the $pK^0_S$ mass spectrum forcollisions in carbon is featureless. The yield of $\Theta^+$ baryons per $h$Ccollision is restricted to be $< 24$\% of the yield per $h$Cu collision.

Search for $\Theta^+(1540)$ emission in hadron--nucleus collisions at 400--700 GeV