No abstract

We investigate theoretical uncertainties and model dependence in the extraction of the nucleon-A(1232) electromagnetic transition amplitudes from the multipole data base. Our starting point is an effective Lagrangian incorporating chiral symmetry, which includes, at the tree level, the pseudovector nucleon Born terms, leading t-channel vector-meson exchanges, and s-and u-channel A exchanges. We express the nucleon-A transition magnetic dipole (MI) and electric quadrupole (E2) amplitudes in terms of two independent gauge couplings at the yNA vertex, and fit these to various multipole data sets. We find a large sensitivity to the method used in unitarizing the amplitude, and extract the E2/M1 ratio (EMR) to be negative, with a magnitude of around 1.5%. The resonant amplitudes in this work are of interest to the test of topical hadron models inspired by QCD: the sign of the EMR, extracted by us, is in accord with that predicted by most realistic models, and its magnitude lies between the predictions of the quark shell model and the Skyrmion model. Finally, our work provides a phenomenologically satisfying unitary amplitude for pion photoproduction off nucleons, which can be used as a realistic starting point for theoretical studies of this process in complex nuclei.

We investigate eta 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 non-resonant pieces. In addition, we consider five resonance contributions in the s-and u-channel; besides the dominant N * (1535), these are: N * (1440), N * (1520), N * (1650) and N * (1710). The amplitudes for the π • 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 χΓηA 1/2 /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 A 1/2 = (97 ± 7) × 10 −3 GeV −1/2 from the old data base, and A 1/2 = (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 eta-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.