EFFECTIVE LAGRANGIAN APPROACH TO THE THEORY OF ETA-PHOTOPRODUCTION IN THE N-ASTERISK(1535) REGION

We investigate η photoproduction in the N*(1535) resonance region within the effective Lagrangian approach (ELA), wherein leading contributions to the amplitude at the tree level are taken into account. These include the nucleon Born terms and the leading t-channel vector meson exchanges as the nonresonant pieces. In addition, we consider five resonance contributions in the s and u channels; in addition to the dominant N*(1535), these are N*(1440), N*(1520), N*(1650), and N*(1710). The amplitudes for the π0 and the η photoproduction near threshold have significant differences, even as they share common contributions, such as those of the nucleon Born terms. Among these differences, the contribution to the η photoproduction of the s-channel excitation of the N*(1535) is the most significant. We find the off-shell properties of the spin-3/2 resonances to be important in determining the background contributions. Fitting our effective amplitude to the available data base allows us to extract the quantity χΓη A1/2/ΓT, characteristic of the photoexcitation of the N*(1535) resonance and its decay into the η-nucleon channel, of interest to precise tests of hadron models. At the photon point, we determine it to be (2.2±0.2)×10-1 GeV-1 from the old data base, and (2.2±0.1)×10-1 GeV-1 from a combination of old data base and new Bates data. We obtain the helicity amplitude for N*(1535)→γp to be A1/2=(97±7)×10-3 GeV-1/2 from the old data base, and A1/2=(97±6)×10-3 GeV-1/2 from the combination of the old data base and new Bates data, compared with the results of the analysis of pion photoproduction yielding 74±11, in the same units. The observed differential cross section is not very sensitive to either the nature of the η-nucleon coupling or to the precise value of the coupling constant; we extract a broad range of values for the ηNN pseudoscalar coupling constant: 0.2≤gη≤6.2 from our analysis of all available data. We predict, in our ELA, the angular distributions for a critical series of experiments at Mainz, and find them to be in good agreement with the preliminary Mainz data. Finally, we discuss implications for future experimental studies with real photons at the Continuous Electron Beam Accelerator Facility (CEBAF) and other emerging medium-energy electron accelerators. Polarization observables, in particular, invite special scrutiny at high precision. © 1995 The American Physical Society.