Evidence for an Exotic<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>S</mml:mi><mml:mo>=</mml:mo><mml:mo>−</mml:mo><mml:mn>2</mml:mn></mml:math>,<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>Q</mml:mi><mml:mo>=</mml:mo><mml:mo>−</mml:mo><mml:mn>2</mml:mn></mml:math>Baryon Resonance in Proton-Proton Collisions at the CERN SPS

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

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