Evidence for a narrow S=1 baryon state with a mass of 1528 MeV at the HERMES experiment

Evidence for a narrow baryon state is found in quasi-real photoproduction off a deuterium target at HERMES through the decay channel pKs0 → pπ+π-. A peak is observed in the pKs0 invariant mass spectrum at 1528 ± 2.6(stat) ±2.1(syst) MeV with a statistical significance of 4 standard deviations. Its intrinsic width is 17 ± 9(stat) ± 3(syst) MeV and its photoproduction cross section is about 100-220 nb. The absence of a similar mass peak in the Λπ+ spectrum suggests that the observed resonance is not a Σ*+, but can be identified with the predicted S = +1 exotic Θ+ (uudds-bar) 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 Θ+ is likely an isoscalar. In the 2004 running, HERMES added a dedicated trigger to enhance the Θ+ signal in the hydrogen data set. In 2005, HERMES will add a recoil detector which will enhance the kinematical acceptance of the spectrometer