Evidence for a narrow |S |= 1 baryon state at a mass of 1528 MeV in quasi-real photoproduction

Abstract Evidence for a narrow baryon state is found in quasi-real photoproduction on a deuterium target through the decay channel pK0S→pπ+π−. A peak is observed in the pK0S invariant mass spectrum at 1528±2.6(stat)±2.1(syst) MeV. Depending on the background model, the naive statistical significance of the peak is 4–6 standard deviations and its width may be somewhat larger than the experimental resolution of σ=4.3–6.2 MeV. This state may be interpreted as the predicted S=+1 exotic Θ + (uudd s ) pentaquark baryon. No signal for an hypothetical Θ++ baryon was observed in the pK+ invariant mass distribution. The absence of such a signal indicates that an isotensor Θ is excluded and an isovector Θ is unlikely.