The photoproduction and electroproduction of the ,Ј mesons on nucleons are investigated within a relativistic chiral unitary approach based on coupled channels. The s-wave potentials for electroproduction and meson-baryon scattering are derived from a chiral effective Lagrangian, which includes Ј as an explicit degree of freedom and incorporates important features of the underlying QCD Lagrangian such as the axial U͑1͒ anomaly. The effective potentials are iterated in a Bethe-Salpeter equation and the cross sections for ,Ј photoproduction and electroproduction from nucleons are obtained. The results for the Ј photoproduction cross section on protons reproduce the appearance of an S 11 resonance around 1.9 GeV observed at ELSA. The inclusion of electromagnetic form factors increases the predicted electroproduction cross sections on the proton, providing a qualitative explanation for the hard form factor of the photocoupling amplitude observed at CLAS.

We include the η ′ in chiral perturbation theory without employing 1/N c counting rules. The method is illustrated by calculating the masses and decay constants of the Goldstone boson octet (π, K, η) and the singlet η ′ up to one-loop order. The effective Lagrangian describing the interactions of the η ′ with the Goldstone boson octet is presented up to fourth chiral order and the loop integrals are evaluated using infrared regularization, which preserves Lorentz and chiral symmetry.

We extend our recent non-perturbative chiral SU(3) coupled channel approach to pion-and photon-induced η-and K-meson production off protons by including all strong and electromagnetic p-wave multipoles. We identify the p-wave amplitudes of the next-to-leading order SU(3) chiral meson-baryon Lagrangian with a coupled channel potential which is iterated to infinite orders in a separable Lippmann-Schwinger equation. Our approach to η-and K-photoproduction introduces no additional free parameters. By adjusting a few finite range parameters and the unknown parameters in the Lagrangian, we are able to simultaneously describe a very large amount of lowenergy data. These include the total and differential cross sections of the π-inducedThe polarization observables measured in η-and K-photoproduction are particularly sensitive to interference terms between the s-and p-wave multipoles. The total cross section data are remarkably well reproduced in all channels. There remain, however, some open questions concerning details of angular distributions and polarization observables.

We discuss the sensitivity of η and η ′ photoproduction near threshold to the gluonic OZI breaking parameters in the U A (1)-extended effective chiral Lagrangian for low-energy QCD. Our coupled-channels analysis hints at a strong correlation between the gluon-induced contributions to the η ′ mass and the low-energy pp → ppη ′ reaction and the near-threshold behaviour of the γp → ηp cross-section.