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

摘要

Evidence for a narrow baryon state is found in quasi-real photoproduction off a deuterium target at HERMES through the decay channel pK p+–. A peak is observed in the pK 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 + spectrum suggests that the observed resonance is not a *+, but can be identified with the predicted S = +1 exotic + (uudd) pentaquark baryon. No signal for an hypothetical ++ baryon was observed in the pK+ invariant mass distribution. The absence of such a signal indicates that + is likely an isoscalar. In the 2004 running, HERMES added a dedicated trigger to enhance the + signal in the hydrogen data set. In 2005, HERMES will add a recoil detector which will enhance the kinematical acceptance of the spectrometer.