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

The gamma p --> p eta' reaction was investigated with the 4 pi 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 gamma p --> p eta' --> p pi(+) pi(-) eta --> p pi(+) pi(-) pi(+) pi(0) were selected. The strong rise of the cross section at threshold and its steep decrease with gamma-energy indicate dominant resonance production. The linear forward rise of the eta' CMS angular distribution is consistent with a coherent excitation of two resonances S-11 and P-11. Assuming dominance of resonance production, the following masses and widths are determined: S-11(M, Gamma) = (1.897 +/- 0.050(-0.002 )(+0.030), 0.396 +/- 0,155 (+0.035)(-0.045 )) GeV and P-11(M, Gamma) = (1.986 +/- 0.026 (+0.010)(-0.030 ), 0.296 +/- 0.100(-0.010 )(+0.060)) GeV, where the first error is of statistical nature while the second represents the systematic error. (C) 1998 Published by Elsevier Science B.V. All rights reserved.