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

The data on hadron--nucleus collisions at 400--700 GeV, collected by the SELEX experiment at Fermilab, are analyzed for formation of the exotic pentaquark baryon $\Theta^+(1540)$. A narrow enhancement near 1539 MeV is observed 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 the peak is near 9 standard deviations. Fitted width of the observed $pK^0_S$ resonance is consistent with being entirely due to experimental resolution, and its intrinsic width is restricted to $\Gamma < 3$ MeV at 90\% CL. The data favor positive rather than negative strangeness for the $pK^0_S$ resonance observed in $h$Cu collisions. At the same time, the $pK^0_S$ mass spectrum for collisions in carbon is featureless. The yield of $\Theta^+$ baryons per $h$C collision is restricted to be $< 24$\% of the yield per $h$Cu collision.