Pentaquark Search at Hermes

Evidence for a narrow baryon state at 1528±2.6(stat)±2.1(syst) MeV is presented in quasi-real photoproduction on a deuterium target through the decay channel pK_S⁰ -> pπ ^ + π ^ - . The statistical significance of the peak in the pK_S⁰ invariant mass spectrum is 4 standard deviations and its extracted intrinsic width Γ = 17 ± 9(stat)±3(syst) MeV. This state may be interpreted as the predicted S=+1 exotic Θ⁺ ( uuddbar s ) pentaquark baryon.