Search for exotic $S=-2$ baryons in $p\overline{p}$ collisions at $\sqrt{s}=1.96\mathrm{TeV}$

A search for a manifestly exotic $S=-2$ baryon state decaying to ${\mathit{\Xi }}^{-}{\pi }^{-}$, and its neutral partner decaying to ${\mathit{\Xi }}^{-}{\pi }^{+}$, has been performed using $220{\mathrm{pb}}^{-1}$ of $p\overline{p}$ collisions at $\sqrt{s}=1.96\mathrm{TeV}$ collected by the Collider Detector at Fermilab. The ${\mathit{\Xi }}^{-}$ trajectories were measured in a silicon tracker before their decay, resulting in a sample with low background and excellent position resolution. No evidence was found for $S=-2$ pentaquark candidates in the invariant mass range of $1600–2100\mathrm{MeV}/c^2$. Upper limits on the product of pentaquark production cross section times its branching fraction to ${\mathit{\Xi }}^{-}{\pi }^{+,-}$, relative to the cross section of the well-established $\mathit{\Xi }(1530)$ resonance, are presented for neutral and doubly negative candidates with $p_T>2\mathrm{GeV}/c$ and $|y|<1$ as a function of pentaquark mass. At $1862\mathrm{MeV}/c^2$, these upper limits for neutral and doubly negative final states were found to be 3.2% and 1.7% at the 90% confidence level, respectively.

Figure 1

Invariant mass spectra from the SVT sample for (a) $\mathit{\Lambda }{\pi }^{\mp }$ candidates that have an associated ${\mathit{\Xi }}^{-}$ track in the SVX II, where the open histogram shows right-sign $\mathit{\Lambda }{\pi }^{-}$ combinations, and the filled histogram shows wrong-sign $\mathit{\Lambda }{\pi }^{+}$ combinations; (b) ${\mathit{\Xi }}^{-}{\pi }^{+}$ combinations, which display a clear $\mathit{\Xi }(1530)$ peak; and (c) ${\mathit{\Xi }}^{-}{\pi }^{-}$ combinations. Arrows mark the mass at $1862\mathrm{MeV}/c^2$, where the NA49 Collaboration reported observing the $\Phi$. The smooth curves represent fits to the spectra and are described in the text.

Figure 2

Invariant mass spectra from the Jet20 sample for (a) $\mathit{\Lambda }{\pi }^{\mp }$ candidates that have an associated ${\mathit{\Xi }}^{-}$ track, where the open (filled) histogram is right-sign $\mathit{\Lambda }{\pi }^{-}$ (wrong-sign $\mathit{\Lambda }{\pi }^{+}$) combinations; (b) ${\mathit{\Xi }}^{-}{\pi }^{+}$ combinations; and (c) ${\mathit{\Xi }}^{-}{\pi }^{-}$ combinations. Arrows mark $1862\mathrm{MeV}/c^2$, and the smooth curves represent fits described in the text.

Figure 3

Reconstructed transverse momentum spectrum of the $\mathit{\Xi }(1530)$. The fit function is described in the text.

Figure 4

Upper limits from SVT data (90% confidence level) for the production cross section of $\Phi$ pentaquarks times the $\Phi \to \mathit{\Xi }\pi$ branching ratios relative to the $\mathit{\Xi }(1530)$ cross section as a function of (a) ${\Phi }^0$ mass, and (b) ${\Phi }^{--}$ mass. The solid line shows the results obtained assuming zero natural width of the pentaquark, and the dashed line shows the results obtained assuming $\Gamma =17\mathrm{MeV}/c^2$. The arrows mark the position of the NA49 peak.

Figure 5

Upper limits from Jet20 data (90% confidence level) for the production cross section of $\Phi$ pentaquarks times the $\Phi \to \mathit{\Xi }\pi$ branching ratios relative to the $\mathit{\Xi }(1530)$ cross section as a function of (a) ${\Phi }^0$ mass, and (b) ${\Phi }^{--}$ mass. The solid (dashed) line shows the results obtained assuming $\Gamma =0(17)\mathrm{MeV}/c^2$. The arrows mark the position of the NA49 peak.