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

TL;DR

This paper reports the observation of a narrow exotic baryon resonance with strangeness -2 and charge -2 in proton-proton collisions at CERN SPS, providing evidence for a new pentaquark candidate.

Contribution

It presents the first evidence for a narrow S = -2, Q = -2 baryon resonance in proton-proton collisions, suggesting a possible exotic pentaquark state.

Findings

Evidence for a narrow Xi- pi- resonance at 1.862 GeV/c^2

Observation of a peak in Xi- pi+ spectrum at the same mass

Enhancements in antibaryon spectra at the same invariant mass

Abstract

Results of resonance searches in the Xi- pi-, Xi- pi+, antiXi+ pi- and antiXi+ 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 ubar). 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 (d s u s dbar). The corresponding antibaryon spectra also show enhancements at the same invariant mass.