Evidence for an Exotic $S=-2$, $Q=-2$ Baryon Resonance in Proton-Proton Collisions at the CERN SPS

Results of resonance searches in the ${\Xi }^{-}{\pi }^{-}$, ${\Xi }^{-}{\pi }^{+}$, ${\overline{\Xi }}^{+}{\pi }^{-}$, and ${\overline{\Xi }}^{+}{\pi }^{+}$ invariant mass spectra in proton-proton collisions at $\sqrt{s}=17.2\mathrm{G}\mathrm{e}\mathrm{V}$ are presented. Evidence is shown for the existence of a narrow ${\Xi }^{-}{\pi }^{-}$ baryon resonance with mass of $1.862\pm 0.002\mathrm{G}\mathrm{e}\mathrm{V}/c^2$ and width below the detector resolution of about $0.018\mathrm{G}\mathrm{e}\mathrm{V}/c^2$. The significance is estimated to be above $4.2\sigma$. This state is a candidate for the hypothetical exotic ${\Xi }_{3/2}^{--}$ baryon with $S=-2$, $I=\frac{3}{2}$, and a quark content of ($dsds\overline{u}$). At the same mass, a peak is observed in the ${\Xi }^{-}{\pi }^{+}$ spectrum which is a candidate for the ${\Xi }_{3/2}^0$ member of this isospin quartet with a quark content of ($dsus\overline{d}$). The corresponding antibaryon spectra also show enhancements at the same invariant mass.

Figure 1

(a) The $p{\pi }^{-}$ invariant mass spectrum for V0 topologies. (b) The $\Lambda {\pi }^{-}$ invariant mass spectrum for ${\Xi }^{-}$ candidates. Curves depict the results from a simulation of the detector response, and shaded areas indicate the range of the selected candidates. The arrows show the nominal $\Lambda$ and $\Xi$ masses. (c),(d) Analogous spectra for $\overline{\Lambda }$ and ${\overline{\Xi }}^{+}$.

Figure 2

Invariant mass spectra after selection cuts for ${\Xi }^{-}{\pi }^{-}$ (a), ${\Xi }^{-}{\pi }^{+}$ (b), ${\overline{\Xi }}^{+}{\pi }^{-}$ (note that the $\overline{\Xi }(1530)°$ state is also visible) (c), and ${\overline{\Xi }}^{+}{\pi }^{+}$ (d). The shaded histograms are the normalized mixed-event backgrounds.

Figure 3

(a) The sum of the ${\Xi }^{-}{\pi }^{-}$, ${\Xi }^{-}{\pi }^{+}$, ${\overline{\Xi }}^{+}{\pi }^{-}$, and ${\overline{\Xi }}^{+}{\pi }^{+}$ invariant mass spectra. The shaded histogram shows the normalized mixed-event background. (b) Background subtracted spectrum with the Gaussian fit to the peak.