Evidence for an exotic S=-2, Q=-2 baryon resonance in proton-proton collisions at the CERN SPS

Results of resonance searches in the $\Xi^{-} \pi^{-}, \Xi^{-} \pi ^{+}, \bar{\Xi}^{+} \pi^{-}$, 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$\pm$0.002 GeV/$c^{2}$ and width below the detector resolution of about 0.018 GeV/$c^{2}$. The significance is estimated to be above 4.2 $\sigma$. This state is a candidate for the hypothetical exotic $\Xi$$^{-}$$^{-}$$\_3$$\_/$$\_2$ baryon with S=-2, I=3/2, and a quark content of $(dsds\bar{u})$. At the same mass, a peak is observed in the $\Xi^{-} \pi ^{+}$ spectrum which is a candidate for the $\Xi$$^{0}$$\_3$$\_/$$\_2$ member of this isospin quartet with a quark content of $(dsus\bar{d})$. The corresponding antibaryon spectra also show enhancements at the same invariant mass. (21 refs).