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

Results of resonance searches in the Ξ - π - , Ξ - π + , Ξ + π - , and Ξ + π + invariant mass spectra in proton-proton collisions at √s = 17.2 GeV are presented. Evidence is shown for the existence of a narrow Ξ - π - 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 above 4.2σ. This state is a candidate for the hypothetical exotic Ξ - - 3 / 2 baryon with S = -2, I = 3/2, and a quark content of (dsdsŪ). At the same mass, a peak is observed in the Ξ - π + spectrum which is a candidate for the Ξ 0 3/2 member of this isospin quartet with a quark content of (dsusd). The corresponding antibaryon spectra also show enhancements at the same invariant mass.