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

TL;DR

The SELEX experiment analyzed 400-700 GeV hadron--nucleus collision data and observed a statistically significant narrow resonance near 1540 MeV consistent with the $ heta^+(1540)$ pentaquark, with a preference for positive strangeness.

Contribution

First observation of a narrow $ heta^+(1540)$ candidate in hadron--nucleus collisions at Fermilab with detailed mass and width analysis.

Findings

Narrow resonance near 1539 MeV with 9 sigma significance.

Intrinsic width of the resonance is less than 3 MeV.

Resonance observed in copper but not in carbon collisions.

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 $pK^0_S$ 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.