Photoproduction of eta '-mesons with the 4 pi-detector SAPHIR

The gamma p --> p eta' reaction was investigated with the 4 pi magnetic spe ctrometer SAPHIR at ELSA with tagged photons in the energy range from 0.9 t o 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 s ection at threshold and its steep decrease with gamma-energy indicate domin ant resonance production. The linear forward rise of the eta' CMS angular d istribution 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 right s reserved.