Observation of an Exotic S = -2, Q = -2 Baryon Resonance in Proton-Proton Collisions at the CERN SPS

Results of resonance searches in the $\Xi^{-} \pi^{-}, \Xi^{-} \pi^{+}, \bar{Xi}^{+} \pi-$ and $\bar{Xi}^{+} \pi^{+}$ invariant mass spectra in proton-proton collisions at $\sqrt{s}=17.2$ GeV are presented. Evidence is shown for the existence of a narrow $\Xi^{-} \pi^{-}$ baryon resonance with mass of 1.862+/-0.002 GeV/c^2 and width below the detector resolution of about 0.018 GeV/c^2. The significance is estimated to be 4.0 sigma. This state is a candidate for the hypothetical exotic $\Xi$$\_3$$\_/$$\_2$$^-$$^-$ baryon with S = -2, I = 3/2 and a quark content of $(d s d s \bar{u})$. At the same mass a peak is observed in the $\Xi^{-} \pi^{+}$ spectrum which is a candidate for the $\Xi$$^0$$\_3$$\_/$$\_2$ member of this isospin quartet with a quark content of $(d s u s \bar{d})$. The corresponding antibaryon spectra also show enhancements at the same invariant mass.