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

Abstract

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 $p{K}_S^0$ 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 $p{K}_S^0$ 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 $p{K}_S^0$ resonance observed in $h$Cu collisions. At the same time, the $p{K}_S^0$ 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.

12 pages, 8 figures