Photoproduction of η′-mesons with the 4π-detector SAPHIR

The γp→pη′ reaction was investigated with the 4π magnetic spectrometer SAPHIR at ELSA with tagged photons in the energy range from 0.9 to 2.6 GeV. On the basis of six million hadronic events 250 events with five tracks due to the reaction chain γp→pη′→pπ+π−η→pπ+π−π+π−π0 were selected. The strong rise of the cross section at threshold and its steep decrease with γ-energy indicate dominant resonance production. The linear forward rise of the η′ CMS angular distribution is consistent with a coherent excitation of two resonances S11 and P11. Assuming dominance of resonance production, the following masses and widths are determined:S11(M,Γ)=(1.897±0.050+0.030−0.002,0.396±0.155+0.035−0.045) GeV andP11(M,Γ)=(1.986±0.026+0.010−0.030,0.296±0.100+0.060−0.010) GeV, where the first error is of statistical nature while the second represents the systematic error.